Fix test loader for dasherized file names.

Commit 036df2e2c00aee broke the test loader so that no test modules were found or loaded, and no tests run.

Before this commit, a checkout of ember-app-kit runs 0 tests. Now it runs 18.

I don't know how to write a failing test for this, because if the test runner is broken, it won't run :cold_sweat: