Problem:    Insufficient testing for reading/writing files.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6257)
            Add "ui_delay" to test_override() and use it for the CTRL-O test.
https://github.com/vim/vim/commit/b340baed9f7fc1c19a0742e2214d54982190c15e

Omit test_override().
Reorder test_writefile.vim to match Vim.

Problem:    Code for modeless selection not sufficiently tested.
Solution:   Add tests.  Move mouse code functionality to a common script file.
            (Yegappan Lakshmanan, closes vim/vim#5821)
https://github.com/vim/vim/commit/515545e11f523d14343b1e588dc0b9bd3d362bc2

Skip termcode tests for now.

Problem:    ModeChanged event not triggered when leaving the cmdline window.
Solution:   Call may_trigger_modechanged(). (closes vim/vim#10791)
https://github.com/vim/vim/commit/c9e8fd6fc7d2027d0645b376d95a6ed51098036c

Code is already present in Nvim. Add some other related missing changes.

Problem:    Insert mode completion not fully tested.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes vim/vim#6041)
https://github.com/vim/vim/commit/f9ab52e155dc13f59b654d754041fe78e17b9074

Cherry-pick test_ins_complete.vim changes from patches 8.2.{0522,0615}.

Problem:    Various code not covered by tests.
Solution:   Add a few more test. (Yegappan Lakshmanan, closes vim/vim#7995)
https://github.com/vim/vim/commit/1f448d906b3c516e5864dc5bae3ddbf3664ee649

Cherry-pick some test_edit.vim changes from patches 8.2.{1022,1432}.
Reorder test_undo.vim to match upstream.

Problem:    Channel and sandbox code not sufficiently tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#5855)
https://github.com/vim/vim/commit/ca68ae13114619df3e4c195b41ad0575516f5ff6

Cherry-pick test_clientserver.vim changes form patch 8.2.0448.

Problem:    Various Normal mode commands not fully tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#5751)
https://github.com/vim/vim/commit/1671f4488105ee12a6a8558ae351436c26ab55fc

Cherry-pick a fix from patch 8.2.3162.
Omit test_iminsert.vim as previous patches to that file are N/A, and
Nvim doesn't support iminsert=2 either, so that test isn't useful.

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes vim/vim#5649)
https://github.com/vim/vim/commit/bc2b71d44a0b90b6aeb3534a76912fccbe5577df

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
https://github.com/vim/vim/commit/ff06f283e3e4b3ec43012dd3b83f8454c98f6639

Fix a mistake in the porting of patch 8.1.0098.
Cherry-pick Test_run_excmd_with_text_locked() from patch 8.2.0270.
Cherry-pick test_gf.vim changes from patch 8.2.0369.
Cherry-pick message change from later patches.

Problem:    Some Normal mode commands not tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#5746)
https://github.com/vim/vim/commit/f5f1e10d0d39890298cdf27f664d466c8872b87e

Omit Test_edit_forbidden(): change reverted in patch 8.2.0369.
Cherry-pick Test_normal_yank_with_excmd() from patch 8.2.0293.
Skip Test_normal_cursorhold_with_count().
Cherry-pick Test_star_register() from patch 8.2.0270.