tests: stabilize compat tests (#7227)

This test had two flaky failure modes:
- pageserver log error for timeline not found: this resulted from
changes for DR when timeline destroy/create was added, but endpoint was
left running during that operation.
- storage controller log error because the test was running for long
enough that a background reconcile happened at almost the exact moment
of test teardown, and our test fixtures tear down the pageservers before
the controller.

Closes: #7224

test_runner/regress/test_compatibility.py

@@ -267,9 +267,10 @@ def test_forward_compatibility(
 
 def check_neon_works(env: NeonEnv, test_output_dir: Path, sql_dump_path: Path, repo_dir: Path):
     ep = env.endpoints.create_start("main")
+    connstr = ep.connstr()
+
     pg_bin = PgBin(test_output_dir, env.pg_distrib_dir, env.pg_version)
 
-    connstr = ep.connstr()
     pg_bin.run_capture(
         ["pg_dumpall", f"--dbname={connstr}", f"--file={test_output_dir / 'dump.sql'}"]
     )
@@ -286,6 +287,9 @@ def check_neon_works(env: NeonEnv, test_output_dir: Path, sql_dump_path: Path, r
     timeline_id = env.initial_timeline
     pg_version = env.pg_version
 
+    # Stop endpoint while we recreate timeline
+    ep.stop()
+
     try:
         pageserver_http.timeline_preserve_initdb_archive(tenant_id, timeline_id)
     except PageserverApiException as e:
@@ -310,6 +314,9 @@ def check_neon_works(env: NeonEnv, test_output_dir: Path, sql_dump_path: Path, r
         existing_initdb_timeline_id=timeline_id,
     )
 
+    # Timeline exists again: restart the endpoint
+    ep.start()
+
     pg_bin.run_capture(
         ["pg_dumpall", f"--dbname={connstr}", f"--file={test_output_dir / 'dump-from-wal.sql'}"]
     )