LibTest: Add the RandomnessSource abstraction

This will be a foundational part of bootstrapping generators: this is
the way they'll get prerecorded values from / record random values into
RandomRuns. (Generators don't get in contact with RandomRuns
themselves, they just interact with the RandomnessSource.)

Userland/Libraries/LibTest/TestResult.h

@@ -17,6 +17,10 @@ enum class TestResult {
 
     // Didn't get through EXPECT(...).
     Failed,
+
+    // Ran out of RandomRun data (in a randomized test, when shrinking).
+    // This is fine, we'll just try some other shrink.
+    Overrun,
 };
 
 // Used eg. to signal we've ran out of prerecorded random bits.