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.
diff --git a/Misc/NEWS b/Misc/NEWS
index f347386..bcfeb4a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -54,6 +54,8 @@
 Tests
 -----
 
+- Issue #10992: Make tests pass under coverage.
+
 - Issue #10826: Prevent sporadic failure in test_subprocess on Solaris due
   to open door files.