Don't validate schema when setting up benchmark tests

GraphQL.js does that too, and it shaves ~0.9s off the execution time:
1.13s -> 0.69s for test_introspection_from_schema.py
0.67s -> 0.21s for test_validate_gql.py

Author: ktosiek

tests/benchmarks/test_introspection_from_schema.py

@@ -6,7 +6,7 @@
 
 
 def test_executing_introspection_query(benchmark, big_schema_sdl):  # noqa: F811
-    schema = build_schema(big_schema_sdl)
+    schema = build_schema(big_schema_sdl, assume_valid=True)
     query = parse(introspection_query.get_introspection_query())
     result = benchmark(lambda: execute(schema, query))
     assert result.errors is None

tests/benchmarks/test_validate_gql.py

@@ -6,7 +6,7 @@
 
 
 def test_validating_introspection_query(benchmark, big_schema_sdl):  # noqa: F811
-    schema = build_schema(big_schema_sdl)
+    schema = build_schema(big_schema_sdl, assume_valid=True)
     query = parse(introspection_query.get_introspection_query())
     result = benchmark(lambda: validate(schema, query))
     assert result == []