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_graceful_shutdown.py

@@ -0,0 +1,25 @@
+import time
+
+from util import Thread
+from postgrest import run
+
+
+def test_graceful_shutdown_waits_for_in_flight_request(defaultenv):
+    "SIGTERM should allow in-flight requests to finish before exiting"
+
+    with run(env=defaultenv, wait_max_seconds=5) as postgrest:
+
+        def sleep():
+            response = postgrest.session.get("/rpc/sleep?seconds=3", timeout=10)
+            assert response.text == ""
+            assert response.status_code == 204
+
+        t = Thread(target=sleep)
+        t.start()
+
+        # Wait for the request to be in-flight before shutting down.
+        time.sleep(1)
+
+        postgrest.process.terminate()
+
+        t.join()