Added regression test for font-lock keywords

Added regression test for font-lock keywords based on files from read-world fsharp applications using the 'ert' and 'faceup' packages.

I randomly picked two GPL:ed F# applications and picked two reasonably looking source files. You can easily replace the files if you have something that is more representative.

By the way, I do not claim copyright to my contribution, the elisp file fsharp-mode-font-apps-tests.el is so simple and follows a standard pattern. (And, of course, the copyright on the F# test files are owned by the authors of the files.)