Restructure test fixtures/utilities

[dominikschubert]

Some cleanup, restructuring of our pytest setup.

Mostly extracting/moving things around.

This PR intentionally tries to avoid touching any logic and doesn't move actual test files. (some tests had to be fixed/removed/skipped though)

[github-actions]

LocalStack integration with Pro

       3 files  ±0         3 suites  ±0   59m 33s ⏱️ - 2m 45s
1 008 tests  - 4     965 ✔️  - 6  43 💤 +2  0 ❌ ±0 
1 304 runs   - 4  1 234 ✔️  - 6  70 💤 +2  0 ❌ ±0 

Results for commit 56d189a. ± Comparison against base commit c31d245.

♻️ This comment has been updated with latest results.

[thrau]

looks great overall, thanks for cleaning up 💯 just a couple of comments and nitpicks about naming.

• generally the goal is to move more complex systems out of localstack.utils, so i would love to see the testing package moved from localstack/utils/ to localstack/ as we discussed.
• i think it would be better to rename fixture_plugin.py and snapshot_plugin.py to fixtures.py, and snapshot.py. i don't think the _plugin adds any useful information. everything in pytest is kinda a plugin.

[thrau]

🚀