feature request: testonly run targets with captured outputs

I was trying to write a weird test, and in my case it would be a lot easier if I could run some commands in the child workspace, but then have the output of that come back to the main workspace and have the main test actually run there. This avoids the need to do a bunch of heavy setup in the child workspace. Something like:

```bazel
default_test_runner(
name = "cquery",
# do something that violates transitive closure, so can't be a genquery
bazel_cmds = ["cquery --output files ..."] ,
)

bazel_integration_run(
name = "child_data_stdout",
test_runner = ":cquery",
testonly = True,
...
)

sh_test(
name = "test",
data = [
":child_data_stdout",
":lots_of",
":local_targets",
],
...
)
```

No files or tests are named. Start by locating the default_test_runner and bazel_integration_run entry points, then inspect how they interact with sh_test and child workspaces. Done should include a focused testonly example proving that child-workspace command output is available to the main test.

Written by the indexing model from the issue text.