lib/math: Add Kunit test suite for gcd()

Add a KUnit test suite for the gcd() function.
This test suite verifies the correctness of gcd() across various
scenarios, including edge cases.

Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Link: https://lore.kernel.org/r/20250203075400.3431330-1-eleanor15x@gmail.com


Signed-off-by: Kees Cook <kees@kernel.org>