Pants: reclassify test utils in BUILD metadata (part 2)

[cognifloyd]

Background

This is another part of introducing pants, as discussed in various TSC meetings.

Related PRs can be found in:

• pantsbuild milestone (which includes PRs since Initialize pantsbuild #5713)
• pantsbuild label (which also includes preparatory PRs that came before adding pantsbuild)

Overview of this PR

I missed 3 BUILD files in #5926, so this reclassifies those sources as well.

This replaces a [3] python_sources targets with python_test_utils. I excluded the test_*.py files in tests/unit directories since those are already covered by python_tests targets.

This PR does not make any functional changes - to Pants, python_sources and python_test_utils are functionally equivalent. So, this is a very minor/small PR.

I also noticed a typo (paths weren't relative but should have been) in st2actions/BUILD, so I included that as well.

Pants documentation

• python_test_utils target
• python_sources target

[bishopbm1]

LGTM!