tests: replace exceptions with assertions whenever possible

Replicates graphql/graphql-js@2616dc7

Author: Cito

tests/execution/test_union_interface.py

@@ -113,7 +113,7 @@ def resolve_pet_type(value, _info, _type):
         return CatType.name
 
     # Not reachable. All possible types have been considered.
-    raise TypeError("Unexpected pet type")
+    assert False, "Unexpected pet type"
 
 
 PetType = GraphQLUnionType("Pet", [DogType, CatType], resolve_type=resolve_pet_type)

tests/execution/test_variables.py

@@ -29,6 +29,7 @@ def parse_serialized_value(value: str) -> str:
 
 def parse_literal_value(ast: ValueNode, _variables=None) -> str:
     assert isinstance(ast, StringValueNode)
+    assert ast.value == "SerializedValue"
     return parse_serialized_value(ast.value)