`cargo xtask test` could take humility target as an argument

When running cargo xtask test on a machine with multiple configured Humility targets, it's necessary to specify which target to run humility commands against. Unfortunately, the -- <extra args> argument cannot be used to pass --target targetname to Humility, as the target argument must come before the subcommand, and the -- <extra args> extra args are passed after the subcommand, and results in the xtask trying to run humility flash -t MYTARGET rather than humility -t MYTARGET flash, which humility dislikes:

eliza@theseus ~/Code/oxide/hubris $ cargo xtask test test/tests-gimletlet/app.toml -- -t gimletlet
    Finished `dev` profile [optimized + debuginfo] target(s) in 0.16s
     Running `target/debug/xtask test test/tests-gimletlet/app.toml -- -t gimletlet`
app.toml has changed; rebuilding all tasks
cleaning ["gimletlet", "test-runner", "test-suite", "test-assist", "test-idol-server", "task-hiffy", "task-idle"]
     Removed 136 files, 198.9MiB total
building crate test-runner
   Compiling abi v0.1.0 (/home/eliza/Code/oxide/hubris/sys/abi)
   Compiling test-runner v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-runner)
   Compiling userlib v0.1.0 (/home/eliza/Code/oxide/hubris/sys/userlib)
   Compiling test-api v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-api)
    Finished `release` profile [optimized + debuginfo] target(s) in 1.07s
target/thumbv7em-none-eabihf/release/test-runner -> target/tests-gimletlet/dist/runner.elf
building crate test-suite
   Compiling abi v0.1.0 (/home/eliza/Code/oxide/hubris/sys/abi)
   Compiling test-suite v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-suite)
   Compiling userlib v0.1.0 (/home/eliza/Code/oxide/hubris/sys/userlib)
   Compiling idol-runtime v0.1.0 (https://github.com/oxidecomputer/idolatry.git?branch=eliza/zerocopy-0.8#06a3c333)
   Compiling test-api v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-api)
   Compiling derive-idol-err v0.1.0 (/home/eliza/Code/oxide/hubris/lib/derive-idol-err)
   Compiling test-idol-api v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-idol-api)
cargo:rerun-if-env-changed=HUBRIS_TASK_CONFIG
    Finished `release` profile [optimized + debuginfo] target(s) in 1.51s
target/thumbv7em-none-eabihf/release/test-suite -> target/tests-gimletlet/dist/suite.elf
building crate test-assist
   Compiling abi v0.1.0 (/home/eliza/Code/oxide/hubris/sys/abi)
   Compiling test-assist v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-assist)
   Compiling userlib v0.1.0 (/home/eliza/Code/oxide/hubris/sys/userlib)
   Compiling test-api v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-api)
    Finished `release` profile [optimized + debuginfo] target(s) in 0.83s
target/thumbv7em-none-eabihf/release/test-assist -> target/tests-gimletlet/dist/assist.elf
building crate test-idol-server
   Compiling test-idol-server v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-idol-server)
   Compiling userlib v0.1.0 (/home/eliza/Code/oxide/hubris/sys/userlib)
   Compiling idol-runtime v0.1.0 (https://github.com/oxidecomputer/idolatry.git?branch=eliza/zerocopy-0.8#06a3c333)
   Compiling test-idol-api v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-idol-api)
    Finished `release` profile [optimized + debuginfo] target(s) in 0.87s
target/thumbv7em-none-eabihf/release/test-idol-server -> target/tests-gimletlet/dist/idol.elf
building crate task-hiffy
   Compiling userlib v0.1.0 (/home/eliza/Code/oxide/hubris/sys/userlib)
   Compiling gateway-messages v0.1.0 (https://github.com/oxidecomputer/management-gateway-service#3d7f1cb0)
   Compiling task-hiffy v0.1.0 (/home/eliza/Code/oxide/hubris/task/hiffy)
   Compiling drv-i2c-types v0.1.0 (/home/eliza/Code/oxide/hubris/drv/i2c-types)
   Compiling idol-runtime v0.1.0 (https://github.com/oxidecomputer/idolatry.git?branch=eliza/zerocopy-0.8#06a3c333)
   Compiling test-api v0.1.0 (/home/eliza/Code/oxide/hubris/test/test-api)
   Compiling drv-i2c-api v0.1.0 (/home/eliza/Code/oxide/hubris/drv/i2c-api)
   Compiling drv-spi-api v0.1.0 (/home/eliza/Code/oxide/hubris/drv/spi-api)
    Finished `release` profile [optimized + debuginfo] target(s) in 2.23s
target/thumbv7em-none-eabihf/release/task-hiffy -> target/tests-gimletlet/dist/hiffy.elf
building crate task-idle
   Compiling userlib v0.1.0 (/home/eliza/Code/oxide/hubris/sys/userlib)
   Compiling task-idle v0.1.0 (/home/eliza/Code/oxide/hubris/task/idle)
    Finished `release` profile [optimized + debuginfo] target(s) in 0.43s
target/thumbv7em-none-eabihf/release/task-idle -> target/tests-gimletlet/dist/idle.elf
linking task 'runner'
linking task 'suite'
linking task 'assist'
linking task 'idol'
linking task 'hiffy'
linking task 'idle'
building crate gimletlet
   Compiling abi v0.1.0 (/home/eliza/Code/oxide/hubris/sys/abi)
   Compiling gimletlet v0.1.0 (/home/eliza/Code/oxide/hubris/app/gimletlet)
   Compiling kern v0.1.0 (/home/eliza/Code/oxide/hubris/sys/kern)
    Finished `release` profile [optimized + debuginfo] target(s) in 2.08s
target/thumbv7em-none-eabihf/release/gimletlet -> target/tests-gimletlet/dist/kernel
flash   = 0x08000000..0x08100000
ram     = 0x20000000..0x20020000
sram1_mac = 0x30000000..0x30010000
sram1   = 0x30010000..0x30020000
sram2   = 0x30020000..0x30040000
sram3   = 0x30040000..0x30048000
sram4   = 0x38000000..0x38010000
bank2   = 0x08100000..0x08200000
Used:
  flash:   0x18000 (9%)
  ram:     0x8500 (25%)
  sram1_mac: 0x0 (0%)
  sram1:   0x0 (0%)
  sram2:   0x0 (0%)
  sram3:   0x0 (0%)
  sram4:   0x0 (0%)
  bank2:   0x0 (0%)
error: Found argument '-t' which wasn't expected, or isn't valid in this context

        If you tried to supply `-t` as a value rather than a flag, use `-- -t`

USAGE:
    humility flash [OPTIONS]

For more information try --help
Error: humility failed

While it is possible to run cargo xtask test on a system with multiple humility targets using the HUMILITY_TARGET environment variable, it might be nice to also support -t as well..

Start at the cargo xtask test command entry point and inspect how HUMILITY_TARGET and extra arguments are handled before invoking humility flash. The change is done when a target such as -t gimletlet is accepted directly and passed before the Humility subcommand; verify the command's behavior with a configured target.