Introduce a shared pattern for python test libraries

[AlanCoding]

Lots of collections have pip requirements that are needed for testing but not for use. Probably most of them.

Right now we maintain the exclude list:

ansible-builder/ansible_builder/requirements.py

Lines 7 to 17 in e531baf

	EXCLUDE_REQUIREMENTS = frozenset((
	# obviously already satisfied or unwanted
	'ansible', 'ansible-base', 'python', 'ansible-core',
	# general python test requirements
	'tox', 'pycodestyle', 'yamllint', 'pylint',
	'flake8', 'pytest', 'pytest-xdist', 'coverage', 'mock',
	# test requirements highly specific to Ansible testing
	'ansible-lint', 'molecule', 'galaxy-importer', 'voluptuous',
	# already present in image for py3 environments
	'yaml', 'pyyaml', 'json',
	))

A better way to configure this would be to allow the collection to specify a separate requirements file for test requirements.

Someone might also be interested in an ansible-builder option to produce an EE that includes test requirements. We would need to separate our exclude list into "always exclude" and "exclude for non-test".

Ping @thedoubl3j

Very much related to #210, and we should think through a pattern that applies equally to both these topics.

[AlanCoding]

If we have a "default" location of build-requirements.txt top-level in a collection, then test-requirements.txt seems like the obvious place for test requirements. Of course, we would need a way to give a special location with the meta/ EE metadata for collections.

Some versions of this pattern have already occurred in collections.