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:    Some mapping code is not fully tested.
Solution:   Add more test cases. (Yegappan Lakshmanan, closes vim/vim#5519)
https://github.com/vim/vim/commit/c2a60ae10e7badad9186de59a9994fc8c9f539e0

Problem:    Missing change to eval.c
Solution:   Also change feedkeys().

https://github.com/vim/vim/commit/5f8a14b9dea094b8bbab94cfc1e8da8e633fbc01

Problem:    'langmap' applies to the first character typed in Select mode.
            (David Watson)
Solution:   Check for SELECTMODE. (Christian Brabandt, closes #572)
            Add the 'x' flag to feedkeys().

https://github.com/vim/vim/commit/25281634cda03ce302aaf9f906a9520b5f81f91e