Add KUnit tests to make sure the macro is working correctly. The unit
tests are put behind the new `RUST_BITFIELD_KUNIT_TEST` Kconfig option.

Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
[acourbot:
- Use a consistent test axis where each test focuses on a single thing.
- Rename members to generic name including range for readability.
- Add test exercising `try_with`.
- Add test checking that unallocated bits are left untouched.
]
Co-developed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Yury Norov <ynorov@nvidia.com>
Link: https://patch.msgid.link/20260606-bitfield-v5-2-b92188820914@nvidia.com
[ Prefixed test suite name with `rust_` as mentioned. Markdown-formatted
  a few comments with Markdown. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

There are 6 individual Rust KUnit test suites (plus the doctests one). All
the tests are compiled unconditionally now, which adds ~200 kB to the
kernel image for me on x86_64. As Rust matures, this bloating will
inevitably grow.

Add Kconfig.test which includes a RUST_KUNIT_TESTS menu, and all
individual tests under it.

As usual, new tests are all enabled if KUNIT_ALL_TESTS=y.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Yury Norov <ynorov@nvidia.com>
Reviewed-by: David Gow <david@davidgow.net>
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260417031531.315281-3-ynorov@nvidia.com
[ Fixed capitalization. Used singular for "API" for consistency.
  Reworded to clarify these are suites and that there exists
  the doctests one (which is the biggest at the moment by
  far). - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>