runtest: also update slow test timeout during `#if` clauses

For a `#if slow` test to be useful, we need the test timeout to be increased.
Without this, the slow section would likely be killed before it finish.

tests/run-tests.py

@@ -1397,6 +1397,7 b' class TTest(Test):'
                     return False
             else:
                 reqs.append(arg)
+        self._detectslow(reqs)
         return self._hghave(reqs)[0]
 
     def _parsetest(self, lines):