lib/tests: add KUnit test for bitops

lib/tests: add KUnit test for bitops

Add a KUnit test suite for the bitops API.

The existing 'lib/test_bitops.c' is preserved as-is because it contains
ad-hoc micro-benchmarks 'test_fns' and is intended to ensure no compiler
warnings from C=1 sparse checker or -Wextra compilations.

Introduce 'lib/tests/bitops_kunit.c' for functional regression testing. It
ports the test logic and data patterns from 'lib/test_bitops.c' to KUnit,
verifying correct behavior across various input patterns and
architecture-specific edge cases using isolated stack-allocated bitmaps.

The following test logic has been ported from test_bitops_startup() in
lib/test_bitops.c:
- set_bit() / clear_bit() / find_first_bit() validation ->
  test_set_bit_clear_bit()
- get_count_order() validation -> test_get_count_order()
- get_count_order_long() validation -> test_get_count_order_long()

Also improve the find_first_bit() test to check the full bitmap length
(BITOPS_LENGTH) instead of omitting the last bit, ensuring the bitmap is
completely empty after cleanup.

Verified on x86_64, i386, and arm64 architectures.

Sample KUnit output:

    KTAP version 1
    # Subtest: bitops
    # module: bitops_kunit
    1..3
        KTAP version 1
        # Subtest: test_set_bit_clear_bit
        ok 1 BITOPS_4
        ok 2 BITOPS_7
        ok 3 BITOPS_11
        ok 4 BITOPS_31
        ok 5 BITOPS_88
    # test_set_bit_clear_bit: pass:5 fail:0 skip:0 total:5
    ok 1 test_set_bit_clear_bit
        KTAP version 1
        # Subtest: test_get_count_order
        ok 1 0x00000003
        ok 2 0x00000004
        ok 3 0x00001fff
        ok 4 0x00002000
        ok 5 0x50000000
        ok 6 0x80000000
        ok 7 0x80003000
    # test_get_count_order: pass:7 fail:0 skip:0 total:7
    ok 2 test_get_count_order
        KTAP version 1
        # Subtest: test_get_count_order_long
        ok 1 0x0000000300000000
        ok 2 0x0000000400000000
        ok 3 0x00001fff00000000
        ok 4 0x0000200000000000
        ok 5 0x5000000000000000
        ok 6 0x8000000000000000
        ok 7 0x8000300000000000
    # test_get_count_order_long: pass:7 fail:0 skip:0 total:7
    ok 3 test_get_count_order_long

[Yury: trim Kconfig help message]

CC: Jesse Brandeburg <jesse.brandeburg@intel.com>
CC: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Ryota Sakamoto <sakamo.ryota@gmail.com>
Signed-off-by: Yury Norov <ynorov@nvidia.com>