test: Support a new `.skip_cpp` test suffix.
test: Support a new `.skip_cpp` test suffix.

Following the example of `.skip_warning_as_error`, this patch
introduces a `.skip_cpp` suffix to skip the generation of `.cpp`
files.

This patch also simplifies the code. Ideally, we would create and
implement a new trait offering an API like `is_cpp_skipped`,
`is_warning_as_error_skipped`, and `normalize` (to remove all
suffixes), but it's a little bit overkill for our needs right now.

eqrion/cbindgen,eqrion/cbindgen,eqrion/cbindgen