libc/tests: add stdbit test framework and unit tests

This adds unit tests for all 70 functions in <stdbit.h>.

I'm sorry for the test framework, but it makes it so I don't
have to write 70 unit test

libc/tests: add stdbit test framework and unit tests

This adds unit tests for all 70 functions in <stdbit.h>.

I'm sorry for the test framework, but it makes it so I don't
have to write 70 unit tests by hand.

Reviewed by: adrian, des
Approved by: markj (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D53660

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