tests: flush in test_isolation (#10658)

## Problem

This test occasionally fails while the test teardown tries to do a
graceful shutdown, because the test has quickly written lots of data
into the pageserver.

Closes: #10654 

## Summary of changes

- Call `post_checks` at the end of `test_isolation`, as we already do
for test_pg_regress -- this improves our detection of issues, and as a
nice side effect flushes the pageserver.
- Ignore pg_notify files when validating state at end of test, these are
not expected to be the same

test_runner/fixtures/neon_fixtures.py

@@ -4988,8 +4988,13 @@ def check_restored_datadir_content(
 
     restored_files = list_files_to_compare(restored_dir_path)
 
+    # pg_notify files are always ignored
+    pgdata_files = [f for f in pgdata_files if not f.startswith("pg_notify")]
+    restored_files = [f for f in restored_files if not f.startswith("pg_notify")]
+
+    # pg_xact and pg_multixact files are optional in basebackup: depending on our configuration they
+    # may be omitted and loaded on demand.
     if pgdata_files != restored_files:
-        # filter pg_xact and multixact files which are downloaded on demand
         pgdata_files = [
             f
             for f in pgdata_files

test_runner/regress/test_pg_regress.py

@@ -261,7 +261,7 @@ def test_isolation(
         pg_bin.run(pg_isolation_regress_command, env=env_vars, cwd=runpath)
 
     # This fails with a mismatch on `pg_multixact/offsets/0000`
-    # post_checks(env, test_output_dir, DBNAME, endpoint)
+    post_checks(env, test_output_dir, DBNAME, endpoint)
 
 
 # Run extra Neon-specific pg_regress-based tests. The tests and their