bpo-33855: Still more edits and minimal tests for IDLE  (GH-7784)

Part 3 of 3, continuing PR #7689. This covers 14 idlelib modules and their tests,
rpc to zoomheight except for run (already done) and tooltip (being done separately).
diff --git a/Lib/idlelib/stackviewer.py b/Lib/idlelib/stackviewer.py
index 400fa63..71cfb78 100644
--- a/Lib/idlelib/stackviewer.py
+++ b/Lib/idlelib/stackviewer.py
@@ -148,5 +148,8 @@
     del sys.last_traceback
 
 if __name__ == '__main__':
+    from unittest import main
+    main('idlelib.idle_test.test_stackviewer', verbosity=2)
+
     from idlelib.idle_test.htest import run
     run(_stack_viewer)