Move V2 test runner integration test into proper location of backend/python folder

[Eric-Arellano]

test_test_integration.py was really an integration test for the V2 Pytest runner, not for the generic V2 test runner. For example, these tests would not make sense in the context of running Java tests. Related code should live near each other to make things more discoverable.

Also, this changes the BUILD for pants_test/rules to create an entry for each test file and to properly include each test's dependencies, which were quite out of date. Having a target per test file makes it easier to keep dependencies up to date and makes it easier to run a specific test suite.

This is prework for #7846.

[jsirois]

My read on this affordance is that it supports legacy targets and the hope is for the irregularity to go away:

1. pants/src/python/pants/build_graph/target.py

   Lines 115 to 120 in ba0132d

   	# The source argument is automatically promoted to the sources argument, and rules should
   	# always use the sources argument; don't error if people used source in their BUILD file.
   	# There doesn't appear to be an easy way to error if a Target subclass explicitly takes
   	# `source` as a named kwarg; it would be nice to error rather than silently swallow the value,
   	# if there were such a way.
   	ignore_params.add('source')

2. source attribute is automatically promoted to sources source attribute is automatically promoted to sources #5908
3. Undocumented feature: https://www.pantsbuild.org/build_dictionary.html

My read could definitely be wrong though - @illicitonion seems like the expert here on intent.

[illicitonion]

I'm largely indifferent to source vs sources; both work, there are guard-rails in place to stop you from using both, and I have no intention of trying to get rid of either...

[illicitonion]

I'm largely indifferent to source vs sources; both work, there are guard-rails in place to stop you from using both, and I have no intention of trying to get rid of either...