runtest: move slow timeout process earlier in the `_hghave` method

runtest: move slow timeout process earlier in the `_hghave` method

Before this changesets, early returns might prevent this logic to apply. It
seem safer to run it sooner.

--- a/tests/run-tests.py	Sun Feb 24 19:56:34 2019 +0100
+++ b/tests/run-tests.py	Sun Feb 24 19:56:08 2019 +0100
@@ -1353,6 +1353,9 @@
 
     def _hghave(self, reqs):
         allreqs = b' '.join(reqs)
+
+        self._detectslow(reqs)
+
         if allreqs in self._have:
             return self._have.get(allreqs)
 
@@ -1374,8 +1377,6 @@
             self._have[allreqs] = (False, stdout)
             return False, stdout
 
-        self._detectslow(reqs)
-
         self._have[allreqs] = (True, None)
         return True, None