languages: Enable grammar loading when compiling with test feature (#19881)

This ensures that `cargo tests -p languages` will not fail with a
confusing error message.

Follow up to #19821

We opted to check the `test` feature flag instead of defining a runtime
flag, because we only want to include the `tree-sitter-*` dependencies
in some cases, which is not possible with a runtime flag.

Co-Authored-by: Thorsten <thorsten@zed.dev>

Release Notes:

- N/A

Co-authored-by: Thorsten <thorsten@zed.dev>