lib: math: Move KUnit tests into tests/ subdir

lib: math: Move KUnit tests into tests/ subdir

This patch is a follow-up task from a discussion stemming from point 3
in a recent patch introducing the int_pow kunit test [1] and
documentation regarding kunit test style and nomenclature [2].

Colocate all kunit test suites in lib/math/tests/ and
follow recommended naming convention for files <suite>_kunit.c
and kconfig entries CONFIG_<name>_KUNIT_TEST.

Link: https://lore.kernel.org/all/CABVgOS=-vh5TqHFCq_jo=ffq8v_nGgr6JsPnOZag3e6+19ysxQ@mail.gmail.com/
Link: https://docs.kernel.org/dev-tools/kunit/style.html
Signed-off-by: Luis Felipe Hernandez <luis.hernandez093@gmail.com>
Acked-by: Nicolas Pitre <npitre@baylibre.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Link: https://lore.kernel.org/r/20241202075545.3648096-2-davidgow@google.com
Signed-off-by: Kees Cook <kees@kernel.org>