Clear log stream before failing with longjmp in fatal log

When an assertion fails in the C++ API, it ultimately invokes a
`Fatal`-level `DeepState_Log()`. This calls `DeepState_Fail()`, which
longjmps. If we don't clear the log buffer here, it won't get cleared by
the usual logic which does so. This causes hanging, unflushed log
messages to persist between tests.

This relationship should eventually be inverted and decoupled.

src/lib/Log.c

@@ -77,6 +77,9 @@ void DeepState_Log(enum DeepState_LogLevel level, const char *str) {
   if (DeepState_LogError == level) {
     DeepState_SoftFail();
   } else if (DeepState_LogFatal == level) {
+    /* `DeepState_Fail()` calls `longjmp()`, so we need to make sure
+     * we clean up the log buffer first. */
+    DeepState_ClearStream(level);
     DeepState_Fail();
   }
 }