test: Avoid freeport() collisions in io tests

It's very unlikely, but it can (and did) happen that both the server and
admin ports have the same number returned from freeport(). This then
leads to a situation where PostgREST will accept the same port in both
cases, because the host "localhost" will allow binding to ipv4 or ipv6
respectively. This will make the IO tests fail.

This change makes sure that the admin port will never be the same as the
server port and thus avoids this problem.

test/io/postgrest.py

@@ -102,7 +102,7 @@ def run(
             env["PGRST_SERVER_UNIX_SOCKET"] = str(socketfile)
             baseurl = "http+unix://" + urllib.parse.quote_plus(str(socketfile))
 
-        adminport = freeport()
+        adminport = freeport(port)
         env["PGRST_ADMIN_SERVER_PORT"] = str(adminport)
         adminurl = f"http://localhost:{adminport}"
 
@@ -169,12 +169,15 @@ def metapostgrest():
         yield postgrest
 
 
-def freeport():
+def freeport(used_port=None):
     "Find a free port on localhost."
-    with contextlib.closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as s:
-        s.bind(("", 0))
-        s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
-        return s.getsockname()[1]
+    while True:
+        with contextlib.closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as s:
+            s.bind(("", 0))
+            s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+            port = s.getsockname()[1]
+            if port != used_port:
+                return port
 
 
 def wait_until_exit(postgrest):