tests: override safekeeper ports in storcon DB (#12410)

## Problem
We persist safekeeper host/port in the storcon DB after
https://github.com/neondatabase/neon/pull/11712, so the storcon fails to
ping safekeepers in the compatibility tests, where we start the cluster
from the snapshot.

PR also adds some small code improvements related to the test failure.

- Closes: https://github.com/neondatabase/neon/issues/12339

## Summary of changes
- Update safekeeper ports in the storcon DB when starting the neon from
the dir (snapshot)
- Fail the response on all not-success codes (e.g. 3xx). Should not
happen, but just to be more safe.
- Add `neon_previous/` to .gitignore to make it easier to run compat
tests.
- Add missing EXPORT to the instruction for running compat tests

.gitignore

@@ -6,6 +6,7 @@
 /tmp_check_cli
 __pycache__/
 test_output/
+neon_previous/
 .vscode
 .idea
 *.swp