Don't try to use a fixed port in test_server_config_default (#8272)

We had a flake regarding this.

(At least on my machine?), linux never randomly assigns an even
numbered port, so probably the flake collided with something external
specifically looking for it?

The find_available_port mechanism is fundamentally racy, of course;
the port could be allocated by someone else immediately, and in
testing I did manage to observe cycle lengths as short as 2 before a
reuse.

Possibly these tests should just have a big hammer
`@retry_failed_test(3)` decorator around them or something, but I am
reluctant to introduce this because I fear we will use it instead of
writing proper tests.

(Most tests don't have as good an excuse for flakiness as needing to
allocate a shared resource in a 16-bit namespace).

edb/testbase/server.py

@@ -3002,23 +3002,10 @@ def get_cases_by_shard(cases, selected_shard, total_shards, verbosity, stats):
     return _merge_results(cases)
 
 
-def find_available_port(max_value=None) -> int:
-    if max_value is None:
-        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
-            sock.bind(("localhost", 0))
-            return sock.getsockname()[1]
-    elif max_value > 1024:
-        port = max_value
-        while port > 1024:
-            try:
-                with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
-                    sock.bind(("localhost", port))
-                    return port
-            except IOError:
-                port -= 1
-        raise RuntimeError("cannot find an available port")
-    else:
-        raise ValueError("max_value must be greater than 1024")
+def find_available_port() -> int:
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+        sock.bind(("localhost", 0))
+        return sock.getsockname()[1]
 
 
 def _needs_factoring(weakly):

tests/test_server_config.py

@@ -2366,7 +2366,7 @@ async def test_server_config_env_03(self):
         "cannot use CONFIGURE INSTANCE in multi-tenant mode",
     )
     async def test_server_config_default(self):
-        p1 = tb.find_available_port(max_value=50000)
+        p1 = tb.find_available_port()
         async with tb.start_edgedb_server(
             extra_args=["--port", str(p1)]
         ) as sd:
@@ -2378,7 +2378,7 @@ async def test_server_config_default(self):
                     """),
                     p1,
                 )
-                p2 = tb.find_available_port(p1 - 1)
+                p2 = tb.find_available_port()
                 await conn.execute(f"""\
                     configure instance set listen_port := {p2}
                 """)