Problem:    Not enough testing for quickfix code.
Solution:   Add more tests. Fix uncovered problem. (Yegappan Lakshmanan,
            closes vim/vim#9839)
https://github.com/vim/vim/commit/9c9be05b17ececb1515a2f41a4dedbf848d3d8b6

Omit Test_helpgrep_vim9_restore_cpo().
Cherry-pick test_quickfix.vim change from patch 8.2.0644.

This is a port of these two patches combined:

vim-patch:8.1.1524: tests are silently skipped

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.
https://github.com/vim/vim/commit/b0f94c1ff34d27d33aa9f96204985ea29c2eb0a1

vim-patch:8.1.1544: some balloon tests don't run when they can

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes vim/vim#4538)  Change the feature check into a command for
            consistency.
https://github.com/vim/vim/commit/b46fecd3454399f8ebdc5055302e4bfc5a10f98b

Omit test_lua.vim: previous patches are N/A
Omit test_memory_usage.vim: previous patches are N/A
Omit test_textprop.vim: previous patches are N/A
Omit test_winbar.vim: previous patches are N/A
Omit test_termcodes.vim: mostly N/A
Skip Test_mouse_positon() because it uses test_setmouse().
Cannot throw error in a Test_nocatch_ test.
Use latest `CheckFeature clipboard_working` for test_quotestar.vim

Problem:    Too many #ifdefs.
Solution:   Graduate FEAT_MBYTE, the final chapter.
https://github.com/vim/vim/commit/30276f2beb248557c6b33cd5418bca8b7084b0a5

MSVC builds don't include it.
Test utf8 and latin only if +iconv is missing.

Problem:    When running :make the output may be in the system encoding,
            different from 'encoding'.
Solution:   Add the 'makeencoding' option. (Ken Takata)

https://github.com/vim/vim/commit/2c7292dc5bbf155fe2192d417363b8c085759cad