luassert uses 3 by default, which is often not enough.

Instead of documenting how to increase it, let's use a more fitting
(sane) default of 100 levels.

This might help to have more information in case of errors, like
mentioned in https://github.com/neovim/neovim/commit/eec529cf9e.

Forgot `return` in eec529cf9e13.

    14:13:04,119 INFO  - # ./test/unit/helpers.lua @ 760: mbyte utf_char2bytes for chars 0xa000 - 0xafff
    14:13:06,307 WARN  - E908: using an invalid value as a String
      /usr/home/quickbuild/buildagent/workspace/root/neovim/pull-requests-automated/.deps/usr/bin/luajit:
      ./test/unit/helpers.lua:459: write() error: 32: Broken pipe
    14:13:06,308 WARN  - stack traceback:
    14:13:06,308 WARN  - 	[C]: in function 'throw'
    14:13:06,308 WARN  - 	...quests-automated/.deps/usr/share/lua/5.1/busted/core.lua:149: in function 'error'
    14:13:06,308 WARN  - 	...ts-automated/.deps/usr/share/lua/5.1/luassert/assert.lua:171: in function 'assert'
    14:13:06,308 WARN  - 	./test/unit/helpers.lua:459: in function 'write'
    14:13:06,308 WARN  - 	./test/unit/helpers.lua:626: in function 'hook'
    14:13:06,308 WARN  - 	./test/unit/helpers.lua:574: in function <./test/unit/helpers.lua:557>
    14:13:06,308 WARN  - 	[C]: in function 'type'
    14:13:06,308 WARN  - 	...d/.deps/usr/share/lua/5.1/busted/outputHandlers/base.lua:57: in function 'copyElement'
    14:13:06,308 WARN  - 	...d/.deps/usr/share/lua/5.1/busted/outputHandlers/base.lua:66: in function 'format'
    14:13:06,308 WARN  - 	...d/.deps/usr/share/lua/5.1/busted/outputHandlers/base.lua:172: in function 'fn'
    14:13:06,308 WARN  - 	...-requests-automated/.deps/usr/share/lua/5.1/mediator.lua:103: in function 'publish'
    14:13:06,308 WARN  - 	...quests-automated/.deps/usr/share/lua/5.1/busted/core.lua:201: in function 'safe'
    14:13:06,308 WARN  - 	...quests-automated/.deps/usr/share/lua/5.1/busted/core.lua:312: in function 'execute'
    14:13:06,308 WARN  - 	...sts-automated/.deps/usr/share/lua/5.1/busted/execute.lua:58: in function 'execute'
    14:13:06,308 WARN  - 	...ests-automated/.deps/usr/share/lua/5.1/busted/runner.lua:197: in function <...ests-automated/.deps/usr/share/lua/5.1/busted/runner.lua:11>
    14:13:06,308 WARN  - 	./.deps/usr/lib/luarocks/rocks/busted/2.0.0-1/bin/busted:3: in main chunk
    14:13:06,308 WARN  - 	[C]: at 0x004041a0
    14:13:06,323 WARN  - Terminated
    14:13:06,325 INFO  - Executing post-execute action...
    14:13:06,526 INFO  - Checking step execute condition...
    14:13:06,526 INFO  - Step execute condition satisfied, executing...
    14:13:06,706 INFO  - Executing pre-execute action...
    14:13:06,706 INFO  - Running step...

Initially done in 28e59cb22, but does not appear to be necessary
anymore.

Uses UV_EEXIST directly, just like UV_ENOENT.

ref #8288

The test.functional.helpers and test.unit.helpers modules now include
all of the public functions from test.helpers, so there is no need to
separately require('test.helpers').

This is where "pure functions" can live, which can be shared by Nvim and
test logic which may not have a running Nvim instance available.

If in the future we use Nvim itself as the Lua engine for tests, then
these functions could be moved directly onto the `vim` Lua module.

closes #6580

Automatically include all "global helper" util functions in the
unit.helpers and functional.helpers and modules.  So tests don't need to
expicitly do:

    local global_helpers = require('test.helpers')