test: negative pgrst_db_pool_available in metrics

Proves the failure on https://github.com/PostgREST/postgrest/issues/4622.

This doesn't require additional test infra, only nginx. Taking advantage
of the `stream {}` context which is also compatible with unix socket
besides TCP.

test/io/config.py

@@ -10,6 +10,7 @@ CONFIGSDIR = BASEDIR / "configs"
 FIXTURES = yaml.load(
     (BASEDIR / "fixtures/fixtures.yaml").read_text(), Loader=yaml.Loader
 )
+NGINX_BIN = shutil.which("nginx")
 POSTGREST_BIN = shutil.which("postgrest")
 SECRET = "reallyreallyreallyreallyverysafe"