IDLE - fix devdanzin test issues

[terryjreedy]

The bug scan found 4 broad issues in idle_test/ Details in full reports or in numbered links below.

26 CONSIDER idle_test/test_autocomplete.py:241 Five tests in the suite cannot fail
71 CONSIDER idle_test/mock_idle.py:48 The shared test stubs cannot express the conditions the guards under test exist for
72 CONSIDER idle_test/test_searchengine.py:279 A test class freezes text.index to a constant, leaving seven mutations of live code undetected
73 CONSIDER idle_test/test_undo.py Four test files exit 0 even when their tests fail

Linked PRs

• gh-155648: In IDLE tests, call unittest.main without exit arg #156249
• [3.15] gh-155648: In IDLE tests, call unittest.main without exit arg (GH-156249) #156251
• [3.14] gh-155648: In IDLE tests, call unittest.main without exit arg (GH-156249) #156252
• [3.13] gh-155648: In IDLE tests, call unittest.main without exit arg (GH-156249) #156253
• gh-155648: Fix IDLE tests that cannot fail #156257
• gh-155648: Write the empty and placeholder IDLE tests #156260