It's sometimes inconvenient and not obvious enough that tests
dependencies were listed in the CI pipeline, and duplicated between
jobs. In order to make sure it's easy enough to install those locally,
and make sure they ain't duplicated between jobs, we better move them
into its own requirements.txt file.

Since requirements.txt is about tests, we also better isolated both
requirements.txt and test_action.py in its own separate directory, the
'tests/' directory.