test(io): move remaining tests in test_io.py to their modules

We had just 3 tests remaining in test_io.py. This commit moves them to
their modules. So we have:

* test_graceful_shutdown.py

* test_zero_downtime.py

* test_pg_internal.py

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>
(cherry picked from commit 02d83d1c01)

test/io/test_pg_internal.py

@@ -0,0 +1,26 @@
+from util import psql_as_superuser
+from postgrest import run
+
+
+def test_listener_query_is_visible_in_pg_stat_activity(defaultenv):
+    "The listener connection should show the LISTEN pgrst statement in pg_stat_activity"
+
+    env = {
+        **defaultenv,
+        "PGRST_DB_CHANNEL_ENABLED": "true",
+        "PGAPPNAME": "listener-query-test",
+    }
+
+    with run(env=env):
+        output = psql_as_superuser(
+            """
+        select query
+        from pg_stat_activity
+        where application_name = 'listener-query-test'
+          and query = 'LISTEN "pgrst"'
+        limit 1;
+        """,
+            capture_output=True,
+        ).strip()
+
+        assert output == 'LISTEN "pgrst"'