tests: fix run-tests "slow test" check

Look for bytes in a set of bytes objects.

tests/run-tests.py

@@ -1265,7 +1265,7 b' class TTest(Test):'
         if ret != 0:
             return False, stdout
 
-        if 'slow' in reqs:
+        if b'slow' in reqs:
             self._timeout = self._slowtimeout
         return True, None