Fix: cleanup yasnippet-tests.el

- add edebug-form-specs
- replace "yas/" with "yas-"
- yas/ert => yas-batch-run-tests
Closes #318: correctly implement YAS--MODES-TO-ACTIVATE

- simpler and got rid of YAS--ALL-PARENTS
- also got rid of YAS/MODE-SYMBOL backward compatibility hack