tests: fix mark_exists() check on postgresql with non-standard psqlrc

I have `\timing` in my `~/.psqlrc` which prints to stdout even when the
table doesn't exist.

pytest_django_test/db_helpers.py

@@ -166,8 +166,7 @@ def mark_exists():
     if db_engine == "postgresql":
         r = run_psql(TEST_DB_NAME, "-c", "SELECT 1 FROM mark_table")
 
-        # When something pops out on std_out, we are good
-        return bool(r.std_out)
+        return r.status_code == 0
 
     if db_engine == "mysql":
         r = run_mysql(TEST_DB_NAME, "-e", "SELECT 1 FROM mark_table")