gh-138669: Increase test coverage for difflib

[JanEricNitschke]

Previously:

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%

Now:

Name             Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------
Lib/difflib.py     677     12    328     16    97%   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
------------------------------------------------------------
TOTAL              677     12    328     16    97%

I thought about also testing the "pat" argument for IS_LINE_JUNK, but decided against it because it is completely undocumented.

Additionally i considered adding the raising tests to the doctests, but decided against that to keep them concise.

• Issue: Increase test coverage in difflib module #138669

[encukou]

I'd add a comment to avoid implying that users can rely on opcodes being the same object.

Otherwise, this looks great! Thank you!

[miss-islington-app]

Thanks @JanEricNitschke for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[miss-islington-app]

Thanks @JanEricNitschke for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-138817 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-138818 is a backport of this pull request to the 3.13 branch.