refactor(integration): replace pytest.skip with pytest markers

[Trinaa]

Proposing to use pytest markers instead of pytest.skip + environment variables for filtering integration tests. This will:

• Allow for more accurate skip rate reporting in test metrics, since some tests are only intended for local execution
• Will reduce cyclomatic complexity in the test cases
• Will likely scale better if more filters are needed in future

As a workaround, I've added continued support for the environment variables in the Makefile.

Note: Tests that are filtered out are noted as 'deselected' in cmd output

Closes: #SYNC-4490

An alternative could be introducing a marker like @pytest.mark.no_ci. We could then leave the pytest.skip + environment variables in place and still report skip rates accurately, but I find this less ideal because we would have multiple mechanisms for filtering tests. This complexity seems unnecessary.

Add-on: I removed the no-root option in the Makefile because we use package-mode = false and should no longer need it according to this warning.

[taddes]

Thanks for doing this @Trinaa ! Not to be too picky, but to pick up from @jrconlin 's comment, I thought it might not hurt to also add a line in the test docs https://github.com/mozilla-services/autopush-rs/blob/master/docs/src/testing.md for clarity. I'll leave it up to you, but I kind of like us being in the habit of adding to the core service docs as well when we have a procedural or technical norm we want to establish.

[taddes]

🚀