Problem: tests: various tests can be improved
Solution: Use string interpolation to concatenate strings in
test_winfixbuf, check for specific errors in assert_fails()
(Yegappan Lakshmanan)
closes: vim/vim#1815197ea879b9b
Cherry-pick Test_file_perm.vim changes from patch 9.0.{0363,0611}.
Skip Test_colornames_assignment_and_unassignment().
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>