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:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
https://github.com/vim/vim/commit/aad222c9c9a1e4fe6ae5a1fe95bb084619be0e65

Partial port as this does not include eval.txt change for listener_add.

Cherry-pick eval.txt changes for:
- bufadd from v8.1.1626:
  https://github.com/vim/vim/commit/5ca1ac373ae62b37152cb6f85916b402eda520a5
- setloclist and setqflist headers from:
  https://github.com/vim/vim/commit/b0d45e7f5354375edd02afafde3bd37dac1515ff

Correct eval.txt typo for settabwinvar method call (matches latest Vim).

Problem:    Old style tests are not nice.
Solution:   Turn two tests into new style. (pschuh, closes vim/vim#1797)

https://github.com/vim/vim/commit/28b238225ae618f63cfe5d3d723120960a941da7