IDLE: Fix devdanzin test issues

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

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

Fixed:
26 CONSIDER idle_test/test_autocomplete.py:241 Five tests in the suite cannot fail
73 CONSIDER idle_test/test_undo.py Four test files exit 0 even when their tests fail

Linked PRs

• gh-156249
• gh-156251
• gh-156252
• gh-156253
• gh-156257
• gh-156260
• gh-156315
• gh-156316
• gh-156317
• gh-157204
• gh-157205
• gh-157206