patch 9.1.1442: tests: Test_diff_fold_redraw() is insufficient

patch 9.1.1442: tests: Test_diff_fold_redraw() is insufficient

Problem:  tests: Test_diff_fold_redraw() is insufficient
          (after v9.1.1439, Christ van Willegen)
Solution: improve the test (Gary Johnson)

The original Test_diff_fold_redraw() function, added 2025-06-08 at patch
9.1.1439, had a bug and didn't do a very good job of testing the fold
behavior.  This new version is simpler and more thorough.

The bug was that it checked the fold state of one window twice instead
of checking both windows.

closes: #17492

Signed-off-by: Gary Johnson <garyjohn@spocom.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>