patch 9.1.0893: No test that undofile format does not regress

patch 9.1.0893: No test that undofile format does not regress

Problem:  No test that undofile format does not regress
Solution: include a sample undofile to make sure we are always able to
          read it

This is so, that we don't unintentionally change the undofile format and
make sure we can load an undo file that has been created by an older
Vim.

closes: #16127

Signed-off-by: Christian Brabandt <cb@256bit.org>