selftests: net: support matching cases by name prefix

While writing tests with a lot more cases I got tired of having
to jump back and forth to add the name of the test to the ksft_run()
list. Most unittest frameworks do some name matching, e.g. assume
that functions with names starting with test_ are test cases.

Support similar flow in ksft_run(). Let the author list the desired
prefixes. globals() need to be passed explicitly, IDK how to work
around that.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240420025237.3309296-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>