test(test): cover Commands::Test routing end to end

The unit tests exercise `is_native_test_expression` directly, so inverting or
dropping the branch that calls it left the suite green while `rtk test -d dir`
went back to `sh: 0: Illegal option -d`.

Run the binary instead: native expressions carry the system `test` exit code,
`!` is neither answered backwards nor allowed to swallow a command, argument
boundaries survive, and a command still reaches the test runner.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>