selftests/run_kselftest.sh: Make each test individually selectable

Currently with run_kselftest.sh there is no way to choose which test
we could run. All the tests listed in kselftest-list.txt are all run
every time. This patch enhanced the run_kselftest.sh to make the test
collections (or tests) individually selectable. e.g.:

$ ./run_kselftest.sh -c seccomp -t timers:posix_timers -t timers:nanosleep

Additionally adds a way to list all known tests with "-l", usage
with "-h", and perform a dry run without running tests with "-n".

Co-developed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>