vim-patch:8.1.{1524,1544}: tests are silently skipped (#19276)

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.
b0f94c1ff3

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.
b46fecd345

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

src/nvim/testdir/test_python3.vim

@@ -1,9 +1,8 @@
 " Test for python 3 commands.
 " TODO: move tests from test87.in here.
 
-if !has('python3')
-  finish
-endif
+source check.vim
+CheckFeature python3
 
 func Test_py3do()
   " Check deleting lines does not trigger an ml_get error.