selftests: clone3: clone3: add case CLONE3_ARGS_NO_TEST

Building selftests/clone3 with clang warns about enumeration not handled
in switch case:

clone3.c:54:10: warning: enumeration value 'CLONE3_ARGS_NO_TEST' not handled in switch [-Wswitch]
        switch (test_mode) {
                ^

Add the missing switch case with a comment.

Fixes: 17a81069 ("selftests: add tests for clone3()")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>