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_zero_downtime.py

@@ -0,0 +1,75 @@
+import time
+
+from util import Thread
+from postgrest import (
+    freeport,
+    run,
+    wait_until_exit,
+)
+
+
+def test_so_reuseport_zero_downtime_handover(defaultenv):
+    "A second PostgREST instance should take over on the same main/admin ports without request failures."
+
+    # set host to _all_ addresses to force port conflict without SO_REUSEPORT
+    # setting to localhost (which is the default)
+    # might allow running multiple instances on the same port
+    # as the name might be resolved to many IP addresses
+    host = "0.0.0.0"
+    port = freeport()
+    admin_port = freeport(used_ports=[port])
+    failures = []
+    # mutable location shared between threads
+    keep_running = {"value": True}
+
+    # 1. Start first PostgREST instance
+    # 2. Start a "client" thread issuing requests in a loop
+    #    remembering all received errors
+    # 3. Start second PostgREST instance on the same port as the first one
+    # 4. Wait a little and terminate the first instance
+    #
+    # We expect the client does not get any errors after stopping the first instance
+    # and seamlessly migrate to the second instance.
+    #
+    # 5. Stop client thread
+    # 6. Stop second PostgREST instance
+    # 7. Verify client did not get any errors
+    with run(
+        env={**defaultenv, "PGRST_SERVER_REUSEPORT": "true"},
+        port=port,
+        host=host,
+        admin_port=admin_port,
+    ) as first:
+
+        def continuously_request():
+            while keep_running["value"]:
+                try:
+                    response = first.session.get("/projects", timeout=1)
+                    assert response.status_code == 200
+                except Exception as exc:
+                    failures.append(exc)
+                    break
+                time.sleep(0.2)
+
+        requester = Thread(target=continuously_request)
+        requester.start()
+
+        try:
+            time.sleep(1)
+            with run(
+                env={**defaultenv, "PGRST_SERVER_REUSEPORT": "true"},
+                port=port,
+                host=host,
+                # we do not set SO_REUSEPORT on admin socket
+                admin_port=freeport(used_ports=[port, admin_port]),
+            ):
+                time.sleep(1)
+                first.process.terminate()
+                wait_until_exit(first, 2)
+
+                time.sleep(1)
+        finally:
+            keep_running["value"] = False
+            requester.join()
+
+    assert failures == []