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:    The write message mentions characters, but it's actually bytes.
Solution:   Change "C" to "B" and "characters" to "bytes".
https://github.com/vim/vim/commit/3f40ce78f5c178d15871bd784ed878c78f0b8a44

Problem:    Not enough cscope code is covered by tests.
Solution:   Add a few test cases. (Dominique Pellé, closes vim/vim#8246)
https://github.com/vim/vim/commit/1e469c72ee2914fcffbcd49258036acdabd6f500

Problem:    Cscope code not fully tested.
Solution:   Add more test cases. (Dominique Pelle, closes vim/vim#5886)
https://github.com/vim/vim/commit/d7ffc0ba8ccb039dd19b255368302d9bfe0e010b

Prep for patch v8.2.0511.

Problem:    Cscope test fails.
Solution:   Update expected text.
https://github.com/vim/vim/commit/d6ec1730bab0ef4a472637ffe5796d1351442308

Problem:    Various comment updates needed, missing white space.
Solution:   Update comments, add white space.
https://github.com/vim/vim/commit/259f26ac2d41ecfb28b82c651b2bfc1edc7c3e29

Ignored (partly) applied patch for src/nvim/po/it.po.

Problem:    Cscope test fails when using shadow directory.
Solution:   Resolve symlink in Vim. (James McCoy, closes vim/vim#3364)
https://github.com/vim/vim/commit/320bf2d85e9e2924d896b3072979598c954922e7

Problem:    No test for ":cscope add".
Solution:   Add a test. (Dominique Pelle, closes vim/vim#3212)
https://github.com/vim/vim/commit/2196bca7377ff245866cc3cee65b0adb48432ac3