devtools/config: jest config updates

Adds a setup file for e2e testing with jest. This
will let us consolidate setup code common to all
e2e tests.

Change-Id: I7f0ee7316b67e91345f7e47e507b5fd93da773c8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/292699
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

devtools/config/e2e-test-setup.ts

@@ -0,0 +1,3 @@
+import { toMatchImageSnapshot } from 'jest-image-snapshot';
+
+expect.extend({ toMatchImageSnapshot });

devtools/config/jest.config.js

@@ -1,5 +1,4 @@
-// eslint-disable-next-line no-undef
-module.exports = {
+let config = {
   preset: 'ts-jest',
   rootDir: '../../',
   globals: {
@@ -9,3 +8,16 @@ module.exports = {
   },
   moduleFileExtensions: ['ts', 'js'],
 };
+
+// eslint-disable-next-line no-undef
+const e2e = process.argv.includes('e2e');
+if (e2e) {
+  config = {
+    ...config,
+    setupFilesAfterEnv: ['<rootDir>/devtools/config/e2e-test-setup.ts'],
+    testEnvironment: 'node',
+  };
+}
+
+// eslint-disable-next-line no-undef
+module.exports = config;