Increase test coverage in difflib module

[JanEricNitschke]

The difflib module currently has 19 untested lines:

Name             Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------
Lib/difflib.py     677     19    328     21    96%   523, 697, 699, 865, 867, 871, 885-886, 974, 978, 1061, 1150->1157, 1170->1162, 1239->1246, 1252->1258, 1261->1238, 1263->1262, 1447, 1514-1516, 1526->1532, 1814-1815, 1981->1984, 2090-2091
------------------------------------------------------------
TOTAL              677     19    328     21    96%

Six of these are fairly easy to test, specifically the caching in difflib.SequenceMatcher, the "delete" and "insert" cases for difflib.Differ.compare, and the error conditions for difflib.get_close_matches and diifflib.restore

Linked PRs

• gh-138669: Increase test coverage for difflib #138670
• [3.14] gh-138669: Increase test coverage for difflib (GH-138670) #138817
• [3.13] gh-138669: Increase test coverage for difflib (GH-138670) #138818