Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua

For "backwards" duration, reltimefloat() should return negative value
like its counterpart reltimestr().

ref bab24a88ab48
ref 06af88cd72ea
ref #10521
fix #10452

- reltimestr(): Produce negative value by comparing the unsigned
  proftime_T value to INT64_MAX.

https://github.com/neovim/neovim/issues/10452#issuecomment-511155132
1. The interfaces of nearly all platforms return uint64_t. INT64_MAX is
   only half of that.
2. Low-level interfaces like this typically define that there is no
   fixed starting point. The only guarantees are that it's (a)
   monotonically increasing at a rate that (b) matches real time.

ref 06af88cd72ea
fix #10452

- define proftime_T as signed integer
- profile_sub(): allow negative result

closes #10452

Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:

1. msgpackparse() will show internal error: hash_add() in case of duplicate
   keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
   expected. Test was still functioning somehow though. Currently fixed.

It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.

Add missing test file.

https://github.com/vim/vim/commit/cb00f039332d3188931035e9d07144546fdea78a

Converted to a lua test. Change the tolerance of the test to avoid false
positives on travis.