patch 9.2.0426: tests: still some flaky screendump tests

patch 9.2.0426: tests: still some flaky screendump tests

Problem:  tests: still some flaky screendump tests
          (James McCoy)
Solution: Replace flaky VerifyScreenDump checks with assert_* assertions
          for Test_visual_block_scroll and Test_scrolloffpad_with_folds,
          and remove the now-unused dump files, mark those tests as
          flaky (which happened previously for screendump tests
          automatically) (Yasuhiro Matsumoto).

fixes:   #20096
related: #20095

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>