No longer provide a feature in test-helper.el

The file isn't a library intended to be loaded with `require'.
Instead `ert-runner' loads it using `load'.  Other packages that
use `ert-runner' also come with a file named test-helper.el and
unfortunately many of those also provide `test-helper', which
leads to conflicts.

Also see rejeep/ert-runner.el#38.
Closes #15.

tests/test-helper.el

@@ -8,6 +8,5 @@
 (require 'undercover nil t)
 (undercover "*.el" (:report-format 'simplecov) (:send-report nil))
 
-(provide 'test-helper)
 ;;; test-helper.el ends here