Documentation: KUnit: Update filename best practices

Based on feedback from Linus[1] and follow-up discussions, change the
suggested file naming for KUnit tests.

Link: https://lore.kernel.org/lkml/CAHk-=wgim6pNiGTBMhP8Kd3tsB7_JTAuvNJ=XYd3wPvvk=OHog@mail.gmail.com/

 [1]
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>