LibTest: Add the REJECT and ASSUME macros

REJECT and ASSUME are useful for filtering out unwanted generated
values. While this is not ideal, it is ocassionally useful and so we
include it for convenience.

The main loop of RANDOMIZED_TEST_CASE runs the test case 100 times, each
time trying to generate a different set of values. Inside that loop, if
it sees a REJECT (ASSUME is implemented in terms of REJECT), it retries
up to 15 times before giving up (perhaps it's impossible or just very
improbable to generate a value that will survive REJECT or ASSUME).

REJECT("Reason for rejecting") will just outright fail, while
ASSUME(bool) is more of an equivalent of a .filter() method from
functional languages.

Userland/Libraries/LibTest/TestSuite.cpp

@@ -103,6 +103,8 @@ static DeprecatedString test_result_to_string(TestResult result)
         return "Completed";
     case TestResult::Failed:
         return "Failed";
+    case TestResult::Rejected:
+        return "Rejected";
     case TestResult::Overrun:
         return "Ran out of randomness";
     default: