Problem:    Test files still use function!.
Solution:   Remove the exclamation mark.  Fix overwriting a function.
https://github.com/vim/vim/commit/1e1153600c0377472d62cc553173fe555ddcf5a7

Some of the changes were already applied previously.

v8.1.0736 made some changes for making some emsgs more specific.
Includes the change for Test_lambda_fails() in test_lambda.vim.
Adjust relevant functionaltests to expect the new emsgs.

This patch has been fully ported in my Blob port PR, but it hasn't been
merged yet, so just use what we need from it for now.

Required for v8.1.1821.

Problem:    Calling a lambda is slower than evaluating a string.
Solution:   Make calling a lambda faster. (Ken Takata, closes vim/vim#5727)
https://github.com/vim/vim/commit/f10806b25090879fdc1a86cc0da2f4f34fd21921

Port "uf_flags" constants from patch 8.2.1054 to sync with Vim.
Port user_func_error() from patch 8.2.0149.
Port Test_lambda_scope() changes from patch 8.1.0736 so that it passes.

Problem:    Testing lambda with timer is slow.
Solution:   Do not test timer accuracy, only that it works. (Daniel Hahler,
            closes vim/vim#4723)
https://github.com/vim/vim/commit/9bc4dde45d45df732953491d0f2c3fd3b10a627e

Problem:    Error when defining a Lambda with index of a function result.
Solution:   When not evaluating an expression and skipping a function call,
            set the return value to VAR_UNKNOWN.
https://github.com/vim/vim/commit/b4518563c73460150344a57879bf5b22cb8b1c77

    From test_alot.vim:
    Found errors in Test_lambda_with_timer():
    First run:
    function RunTheTest[35]..Test_lambda_with_timer line 19: Expected True but got 0
    Second run:
    function RunTheTest[35]..Test_lambda_with_timer line 19: Expected True but got 0

previously: #9220

- Timer tests are less reliable on Travis CI macOS 10.12/10.13.
  ref #6829
  ref e39dade80b02
  ref de13113dc16e
  ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
  > We don't guarantee that a X ms timer is triggered during Y ms sleep
  > for any X<Y, though I would expect the load to be really bad for this
  > to happen with X=10ms, Y=40ms.
- Call test_garbagecollect_now(), as Vim does.

- window_split_tab_spec.lua: Put cursor at bottom of :terminal buffer so
  that it follows output.
- inccommand_spec.lua: Increase timeout to allow 2nd retry.
- Timer tests are less reliable on Travis CI macOS 10.12/10.13.
  ref #6829
  ref e39dade80b02
  ref de13113dc16e
  ref https://github.com/neovim/neovim/pull/9095#issuecomment-429603452
  > We don't guarantee that a X ms timer is triggered during Y ms sleep
  > for any X<Y, though I would expect the load to be really bad for this
  > to happen with X=10ms, Y=40ms.