This makes it easier to enable all KUnit fragments.

Adding 'if !KUNIT_ALL_TESTS' so individual tests can not be turned off.
Therefore if KUNIT_ALL_TESTS is enabled that will hide the prompt in
menuconfig.
Reviewed-by: NDavid Gow <davidgow@google.com>
Signed-off-by: NAnders Roxell <anders.roxell@linaro.org>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>