tests: increase test_pg_regress and test_isolation timeouts (#8418)

## Problem

These tests time out ~1 in 50 runs when in debug mode.

There is no indication of a real issue: they're just wrappers that have
large numbers of individual tests contained within on pytest case.

## Summary of changes

- Bump pg_regress timeout from 600 to 900s
- Bump test_isolation timeout from 300s (default) to 600s

In future it would be nice to break out these tests to run individual
cases (or batches thereof) as separate tests, rather than this monolith.

test_runner/regress/test_pg_regress.py

@@ -117,7 +117,7 @@ def post_checks(env: NeonEnv, test_output_dir: Path, db_name: str, endpoint: End
 
 # Run the main PostgreSQL regression tests, in src/test/regress.
 #
-@pytest.mark.timeout(600)
+@pytest.mark.timeout(900)  # Contains many sub-tests, is slow in debug builds
 @pytest.mark.parametrize("shard_count", [None, 4])
 def test_pg_regress(
     neon_env_builder: NeonEnvBuilder,
@@ -186,6 +186,7 @@ def test_pg_regress(
 
 # Run the PostgreSQL "isolation" tests, in src/test/isolation.
 #
+@pytest.mark.timeout(600)  # Contains many sub-tests, is slow in debug builds
 @pytest.mark.parametrize("shard_count", [None, 4])
 def test_isolation(
     neon_env_builder: NeonEnvBuilder,