Changed "VimTestFailure" highlight, link to Error.

I think, you should delegate the colors of syntax highlight to the default
highlighting groups. Because, if you don't, these colors may conflict to
the colorscheme that user select.

But, I have no idea that you should use syntax group for the "Success" test
result...
Merge pull request #8 from AmaiSaeta/fix/error-highlight

 Changed "VimTestFailure" highlight, link to Error.