Added test files with comments

Added default_test.rb and support/helpers.rb files to the cookbook. This serves two purposes:

1) Give people new to minitest-handler an idea of how their test files should be structured, using code comments as examples.

2) Removes false positive file-missing errors that appear when minitest-handler is in the run_list.
Fixed lack of a default empty test in the default_test.rb file.
Merge pull request #35 from charlesjohnson/master

Add test files to minitest-handler cookbook