[RFC] Update busted to version 2.

Busted 1.x doesn't seem to be supported anymore (no 1.x branch on Github), so I think we have to update sooner or later. I am not sure why the changes to the test files are necessary (or rather, why it worked before), maybe someone can shed light on that?

Also (and this was why I originally tried this) this fixes #1031 (and obsoletes #1092). But (and I only thought about this after I had already created these commits) the `color_terminal` output from #983 won't work with this anymore.

From https://github.com/Olivine-Labs/busted/blob/master/busted/outputHandlers/utfTerminal.lua#L48-L50: a traceback is printed in the default `utfTerminal` output handler if verbose is set, so I changed this to be the default (for local builds as well):

```
Failure → /home/florian/Projects/neovim/test/unit/fileio_spec.lua @ 19
file_pat functions file_pat_to_reg_pat returns ^path$ regex for literal path input
/home/florian/Projects/neovim/test/unit/fileio_spec.lua:20: Expected objects to not be the same. Passed in:
(string) '^path$'
Did not expect:
(string) '^path$'

stack traceback:
	/home/florian/Projects/neovim/test/unit/fileio_spec.lua:19: in function </home/florian/Projects/neovim/test/unit/fileio_spec.lua:12>
```

If there are no errors, the output is the same as non-verbose. Output handlers other than `utfTerminal` and `plainTerminal` don't use the verbose flag.

Would this be an acceptable replacement for `color_terminal` for now? An improved output handler could be sent as a PR to busted, maybe it would even get into 2.0.