Add gitlab_schema.gql warning in integration spec

- A missing gitlab_schema.graphql file can cause
  unrelated and hard to debug issues.
- This can be overwritten with GL_IGNORE_WARNINGS=1

config/helpers/check_frontend_integration_env.js

+const fs = require('fs');
+const isESLint = require('./is_eslint');
+
+const GRAPHQL_SCHEMA_PATH = 'tmp/tests/graphql/gitlab_schema.graphql';
+const GRAPHQL_SCHEMA_JOB = 'bundle exec rake gitlab:graphql:schema:dump';
+
+const shouldIgnoreWarnings = JSON.parse(process.env.GL_IGNORE_WARNINGS || '0');
+
+const failCheck = (message) => {
+  console.error(message);
+
+  if (!shouldIgnoreWarnings) {
+    process.exit(1);
+  }
+};
+
+const checkGraphqlSchema = () => {
+  if (!fs.existsSync(GRAPHQL_SCHEMA_PATH)) {
+    const message = `
+ERROR: Expected to find "${GRAPHQL_SCHEMA_PATH}" but file does not exist. Try running:
+
+    ${GRAPHQL_SCHEMA_JOB}
+`;
+
+    failCheck(message);
+  }
+};
+
+const check = () => {
+  if (isESLint(module)) {
+    return;
+  }
+
+  checkGraphqlSchema();
+};
+
+module.exports = check;

jest.config.integration.js

+const checkEnvironment = require('./config/helpers/check_frontend_integration_env');
 const baseConfig = require('./jest.config.base');
 
+checkEnvironment();
+
 module.exports = {
   ...baseConfig('spec/frontend_integration', {
     moduleNameMapper: {

spec/frontend_integration/test_helpers/mock_server/graphql.js

 import { buildSchema, graphql } from 'graphql';
+import { memoize } from 'lodash';
 
-/* eslint-disable import/no-unresolved */
-// This rule is disabled for the following line.
 // The graphql schema is dynamically generated in CI
 // during the `graphql-schema-dump` job.
-import gitlabSchemaStr from '../../../../tmp/tests/graphql/gitlab_schema.graphql';
-/* eslint-enable import/no-unresolved */
+// eslint-disable-next-line global-require, import/no-unresolved
+const getGraphqlSchema = () => require('../../../../tmp/tests/graphql/gitlab_schema.graphql');
 
-const graphqlSchema = buildSchema(gitlabSchemaStr.loc.source.body);
 const graphqlResolvers = {
   project({ fullPath }, schema) {
     const result = schema.projects.findBy({ path_with_namespace: fullPath });
@@ -21,6 +19,7 @@ const graphqlResolvers = {
     };
   },
 };
+const buildGraphqlSchema = memoize(() => buildSchema(getGraphqlSchema().loc.source.body));
 
 export const graphqlQuery = (query, variables, schema) =>
-  graphql(graphqlSchema, query, graphqlResolvers, schema, variables);
+  graphql(buildGraphqlSchema(), query, graphqlResolvers, schema, variables);