lnt importreport and/or lnt submit seem to disregard the test suite specified

Nobody has claimed this yet.

Reproducer:

```
# Create an instance and run a server
lnt create my-instance
cat < my-instance/schemas/libcxx.yaml
format_version: '2'
name: libcxx
metrics:
- name: execution_time
type: Real
display_name: Execution Time
unit: seconds
unit_abbrev: s
run_fields:
- name: git_order
order: true
machine_fields:
- name: hardware
- name: os
EOF
lnt runserver my-instance

cat < x.lnt
foo.execution_time 1
bar.execution_time 2
EOF
lnt importreport --testsuite libcxx --order 1 --run-info git_order=99 --machine my-machine x.lnt x.json
lnt submit --testsuite libcxx http://localhost:8000/db_default/submitRun x.json
```

Based on this, I think the problem is in `lnt submit`, which seems to ignore the provided test suite.

Start by running the provided reproducer with the lnt importreport and lnt submit entry points, then trace how --testsuite is handled by each command. Compare the generated x.json and submitted run to confirm that the libcxx test suite is preserved; done means lnt submit uses the specified suite instead of disregarding it.

Written by the indexing model from the issue text.