Mock 100% coverage (GH-13045)

This was achieved by:
* moving many pass statements in tests onto their own lines, so they pass line coverage and can match an easy ignore pattern if branch coverage is added later.
* removing code that cannot be reached.
* removing long-disabled tests.
* removing unused code.
* adding tests for uncovered code

It turned out that removing `if __name__ == '__main__'` blocks that run unittest.main() at the bottom of test files was surprisingly contentious, so they remain and can be filtered out with an appropriate .coveragerc.

Lib/unittest/test/testmock/testsealable.py

@@ -3,15 +3,10 @@ from unittest import mock
 
 
 class SampleObject:
-    def __init__(self):
-        self.attr_sample1 = 1
-        self.attr_sample2 = 1
 
-    def method_sample1(self):
-        pass
+    def method_sample1(self): pass
 
-    def method_sample2(self):
-        pass
+    def method_sample2(self): pass
 
 
 class TestSealable(unittest.TestCase):