fix color handling in test runner

[Tyl13]

When using CliRunner.invoke(), the isolation function was not mocking _compat.should_strip_ansi
in addition to the utils.should_strip_ansi. The _compat version is only called when on Windows machines which is why it was missed. Also changed the test_testing.py::test_with_color to no longer skip when on windows. This will ensure that future changes to the testing.py modules will not cause a regression back to previous behavior.

fixes #2705

Ensure each step in CONTRIBUTING.rst is complete, especially the following:

• Add tests that demonstrate the correct behavior of the change. Tests
  should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.