refactor: make test_parsing_kernel_version() private

[SteveLauC]

What does this PR do

The test function test_parsing_kernel_version() is public, which is unnecessary since it is under attribute #[test]. Also, it causes the CI failure in PR #2500:

error: missing documentation for a function
   --> src/features.rs:95:5
    |
95  |     pub fn test_parsing_kernel_version() {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
note: the lint level is defined here
   --> src/lib.rs:121:12
    |
121 |     #[deny(missing_docs)]
    |            ^^^^^^^^^^^^

error: could not compile `nix` (lib test) due to 1 previous error

so let's make it private.

Checklist:

• [x] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API