kselftest: add support for skipped tests

Update the kselftest framework to allow client drivers to
specify that some tests were skipped.

Signed-off-by: Timur Tabi <timur@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210214161348.369023-3-timur@kernel.org