lib/kunit: Strip "_test" or "_kunit" suffix from subtest names

If a user (an IGT test) doesn't provide a subtest name when calling
igt_kunit() then we now use the requested kernel module name as IGT
subtest name.  Since names of kunit test modules usually end with a
"_test" or "_kunit" suffix, those parts of the names don't carry any
useful information.  Strip those suffixes from IGT subtest names.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>