Load Jest test environment with .env.sample vars

Next isn't instantiated by Jest, so we need to explicitly tell it
where to get env vars. Using .env.sample as I don't see a need for
more specialised .env.test or other file for our values which are
all test strings.

vercel/next.js#16443

setupTests.js

@@ -1 +1,4 @@
 import '@testing-library/jest-dom/extend-expect'
+
+import dotenv from 'dotenv'
+dotenv.config({ path: '.env.sample' })