The new oldtest directory is in test/old/testdir. The reason for this is
that many tests have hardcoded the parent directory name to be
'testdir'.

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

These versions of python has reached End-of-life. getting rid
of python2 support removes a lot of logic to support two
incompatible python versions in the same version.

Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.
https://github.com/vim/vim/commit/3f4f3d8e7e6fc0494d00cfb75669a554c8e67c8b

test_prompt_buffer.vim already had all the changes, except
Test_prompt_garbage_collect().

vim-patch:8.1.{2031,2421}

Partially reverts #11647. Replaces #11662

The old implementation was removed without clear motivation. The "term option"
hackaround added in its place is neither shorter nor simpler.

The new behavior breaks even init.vim that expliticly check against it:

    if exists('&termencoding')
      set termencoding=utf-8
    endif

There was nothing wrong with the 0.4.x behavior. Empty &tenc has
indicated that the &enc value should be used for all the history of
Nvim. Ignoring setting the option is the expected behavior for Vim
versions that does not support the option (and Nvim is such a version)

'tenc' was also irrelevant to the Test_unicode python3 test.
The reason this has to be disabled is that neovim can't change
internal 'encoding'

Problem:    Test88 is old style.
Solution:   Turn into a new style test. (Yegappan Lakshmanan, closes vim/vim#5347)
https://github.com/vim/vim/commit/213ed008bbcd9fe0d3329b17f5f4af0169e448ff

'test_conceal.vim' requires +conceal and +terminal so it is N/A.

Problem:    Python 3 unicode test still fails on MS-Windows.
Solution:   Do not set 'encoding' to "euc-tw" on MS-Windows.
https://github.com/vim/vim/commit/7fc4785ea19306b7e94beb61f226cf40c32b1aba

'termencoding' option was removed in abaabd1d03fd723630f6addeadee9928faa4cdde
but some plugins check its value.

Problem:    Python 3 unicode test fails on MS-Windows.
Solution:   Do not set 'encoding' to "debug" on MS-Windows.
https://github.com/vim/vim/commit/955f4e6f36ea009b90803d12a62108c94f446778