Debugging an ignored Rust test whose code lives in a nested module (e.g.
`variant_get::test::get_complex_variant`) sometimes exited immediately
with "0 tests matched", When the test name itself came from tree-sitter
runnable capture, which sees only the bare function identifier. So
`--exact get_complex_variant` filtered out the actual test.

The fix is to only append `--exact` when the name contains `::`.
rust-analyzer's `experimental/runnables` produces qualified paths and
keeps the disambiguation introduced in #43110, and the runnables from
tree-sitter no longer break for nested modules.

Closes #51810.

Release Notes:

- Fixed debugging Rust tests in nested modules sometimes immediately
exiting with "0 tests matched".