Problem:    Lua test fails with older Lua version.
Solution:   Adjust expected error messages. (closes #6444)

Problem:    Some other tests fail.
Solution:   Adjust tests for different assert_fails() behavior.

Problem:    Insufficient test coverage for Lua.
Solution:   Add tests. (Yegappan Lakshmanan, closes #6368)  Fix uncovered
            memory leak.  Avoid unnecessary copy/free.

Problem:    Lua: cannot use table.insert() and table.remove().
Solution:   Add the list functions. (Prabir Shrestha, closes #6353)

Problem:    Lua arrays are zero based.
Solution:   Make Lua arrays one based. (Prabir Shrestha, closes #6347)
            Note: this is not backwards compatible.

Problem:    Not so easy to pass a lua function to Vim.
Solution:   Convert a Lua function and closure to a Vim funcref. (Prabir
            Shrestha, closes #6246)

Problem:    Not easy to require Lua modules.
Solution:   Improve use of Lua path. (Prabir Shrestha, closes #6098)

Problem:    Not easy to call a Vim function from Lua.
Solution:   Add vim.call() and vim.fn(). (Prabir Shrestha, closes #6063)

Problem:    Heredoc in scripts does not accept lower case marker.
Solution:   Allow lower case only in non-Vim scripts. (Ken Takata,
            closes #6019)

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)