Flesh out tests for `wildignore_to_regexp`
Flesh out tests for `wildignore_to_regexp`

To make this function more robust, and to make sure I understood it the
right way, I'm adding some more tests. I've used the table in the docs
[1] as a starting point, but I've only focused on the recommended list
of patterns:

> In short, in general you want to use patterns like:
  - `foo`: to match a file at any level.
  - `*.foo`: to match a file with an extension at any level.
  - `*/foo`: to match a file or directory at any level.
  - `*/foo/*`: to match a directory at any level.

[1] https://github.com/wincent/command-t/blob/915048e6aed264dd7bf68e29ba485ade4c44f845/doc/command-t.txt#L1120-L1135

yrro/command-t,wincent/command-t,wincent/command-t,wincent/command-t,yrro/command-t