leak tests: do a better job skipping if uss isn't available. Fixes #485

Author: jamadden

src/greenlet/tests/test_leaks.py

@@ -365,6 +365,11 @@ def _check_untracked_memory_thread(self, deallocate_in_thread=True):
         # Like the above test, but what if there are a bunch of
         # unfinished greenlets in a thread that dies?
         # Does it matter if we deallocate in the thread or not?
+
+        # First, make sure we can get useful measurements. This will
+        # be skipped if not.
+        self.get_process_uss()
+
         EXIT_COUNT = [0]
 
         def f():