Fix AC::TemplateAssertions instance variables not resetting.

Fixes https://github.com/rails/rails/issues/16119.
Merge pull request #16234 from tgxworld/fix_template_assertion_for_integration_test

Fix AC::TemplateAssertions instance variables not resetting.
`create` return an active record object with erros instead of false when validation fails. [skip ci]
Merge pull request #16237 from tonytonyjan/patch

`create` method returns an active record object with errors instead of false when validation fails. [skip ci]
specify N=0 to prevent parallel tests (just run tests on the main process)
Use PROCESS_COUNT to define the number of parallel executors
Split Action Pack tests in a new job on travis

This will make easier to compare run time between Ruby 2.1 and trunk