[RFC] Fix functional tests failing on OSX.

See https://github.com/neovim/neovim/issues/1519 for failure report.
```
Cause    : In OSX, /tmp is a symbolic link to /private/tmp, which causes
           expected and got results different because of implicit
           resolution.
Solution : Resolve path before setting expected value.
```
Fixes #1519.

@rainerborene I add this as a quick fix, to have functional tests passing on master. If you later want to provide another PR improving on this, it's up to you.