Do not provide test-helper feature

"test-helper.el" isn't a library intended to be loaded with `require`.
Instead it is loaded with `load`.  Because of that, it is confusing to
provide a feature using `provide` anyway.  And because around 70 other
packages come with a file named "test-helper.el" and several dozen of
those still provide the feature `test-helper`, this also leads to a,
admittedly somewhat theoretical, feature conflict.

This is also discussed at https://github.com/rejeep/ert-runner.el/issues/38.

test/test-helper.el

@@ -14,5 +14,4 @@
 (require 'webpaste (f-expand "webpaste" webpaste-test--root-path))
 
 
-(provide 'test-helper)
 ;;; test-helper.el ends here