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_makeencoding.vim

@@ -4,8 +4,7 @@ source shared.vim
 
 let s:python = PythonProg()
 if s:python == ''
-  " Can't run this test.
-  finish
+  throw 'Skipped: python program missing'
 endif
 
 let s:script = 'test_makeencoding.py'