fix: force timezone to be UTC for tests (#38215)

react-calendar-heatmap's output depends on the timezone, which means
that snapshots can fail if the timezone changes.  This sets the timezone
as UTC during client tests to avoid that problem.

client/jest-timezone-setup.js

@@ -0,0 +1,3 @@
+module.exports = async () => {
+  process.env.TZ = 'UTC';
+};

client/jest.config.js

@@ -12,6 +12,7 @@ module.exports = {
   globals: {
     __PATH_PREFIX__: ''
   },
+  globalSetup: './jest-timezone-setup.js',
   verbose: true,
   transform: {
     '^.+\\.js$': '<rootDir>/jest.transform.js'

client/jest.test.js

@@ -0,0 +1,6 @@
+/* global expect */
+describe('Timezones', () => {
+  it('should always be UTC', () => {
+    expect(new Date().getTimezoneOffset()).toBe(0);
+  });
+});