runtest: extract the logic that update timeout for slow tests

We want to use the "slow" test logic not only for "#require" clauses but also in
"#if" clauses. For this to be useful we need the logic to bump the timeout in at
least two spots. The first step it to factor it out.

tests/run-tests.py

@@ -1374,12 +1374,16 b' class TTest(Test):'
             self._have[allreqs] = (False, stdout)
             return False, stdout
 
-        if b'slow' in reqs:
-            self._timeout = self._slowtimeout
+        self._detectslow(reqs)
 
         self._have[allreqs] = (True, None)
         return True, None
 
+    def _detectslow(self, reqs):
+        """update the timeout of slow test when appropriate"""
+        if b'slow' in reqs:
+            self._timeout = self._slowtimeout
+
     def _iftest(self, args):
         # implements "#if"
         reqs = []