`./mach test-unit` doesn't work with `package/test(.rs)` argument scheme

[mukilan]

For example, ./mach test-unit style/stylist says there are no tests to run:

Summary [   0.001s] 0 tests run: 0 passed, 809 skipped
error: no tests to run
(hint: use `--no-tests` to customize)

It seems this was broken by https://github.com/servo/servo/pull/39897 where params was added as catch-all parameter to ./mach test-unit. This now overrides the existing test_name catch-all parameter: https://github.com/servo/servo/blob/07935679525d91ab1a86f1ff5b0139a465c02013/python/servo/testing_commands.py#L157

Since test_name is now always empty, the logic to infer package & test name from package/test syntax now has no effect.

I'm not sure if the package/test syntax is useful, so removing it also might be a solution.

[jschwe]

You can do the same with -p <package_name as usual in cargo projects, right?

[mukilan]

Yes, the -p package_name still works.