Issue #10992: make tests pass when run under coverage.

Various tests fail when run under coverage. A primary culprit is refcount tests
which fail as the counts are thrown off by the coverage code. A new decorator
-- test.support.refcount_test -- is used to decorate tests which test refcounts
and to skip them when running under coverage. Other tests simply fail because
of changes in the system (e.g., __local__ suddenly appearing).

Thanks to Kristian Vlaardingerbroek for helping to diagnose the test failures.

Lib/test/test_sys.py

@@ -303,6 +303,7 @@ class SysModuleTest(unittest.TestCase):
             self.assertEqual(sys.getdlopenflags(), oldflags+1)
             sys.setdlopenflags(oldflags)
 
+    @test.support.refcount_test
     def test_refcount(self):
         # n here must be a global in order for this test to pass while
         # tracing with a python function.  Tracing calls PyFrame_FastToLocals