rust: bitfield: Add KUnit tests for bitfield

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>

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