patch 9.1.1995: tests: flaky test_cmdline.vim causes test failures

patch 9.1.1995: tests: flaky test_cmdline.vim causes test failures

Problem:  Data races between test buffers are currently addressed by
          relying on arbitrary execution delays or blissfully dismissed.
Solution: Prefer more deterministic synchronisation between test buffers
          for test cases that generate screendump files with expected
          results (Aliaksei Budavei)

closes: #18920

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>