Merge all unit test files

All these test files have gone out of hand.  A single test library
provides a number of advantages:

- Faster loading
- Easier interactive testing
- Easier navigation in the test suite
- Better error checking with Flycheck
Use f.el to determine the load file of a checker
Add helper to create buffers from files
Only test cfengine if the mode is available
Add tests for flycheck-autoloads-file-p
Simplify some test code
Properly kill modified temporary buffers visiting files