common: add test for connection with keyFilePath.

I need to work on less scary PRs. Sorry! Will annotate.
Filename represents a path to a file. A path could represent anything on the other hand. See standard library to see distinction between path and filenames. http://nodejs.org/api/fs.html

It's much more explicit to use *name for files and symlinks, whereas path terminology is favoured if you don't need a distinction.