Checker: Test files should not just end in .js

[tkissing]

The "test" folder often needs to contain files that are valid JavaScript but not valid tests (mocks, "karma.main.js"-kinda files, test-related helpers, etc).
Having something unique in the names of test files ('.spec.js', '.tap.js', etc) makes globbing much easier and keeps test-runner configuration simple.
It also makes it easier to apply different linting rules to different types of files independent of the folder they are in.