Bug: Memory leak in yarn test

[kulla]

 node --expose-gc ./node_modules/.bin/jest --runInBand --logHeapUsage                            
 PASS  __tests__/auth.ts (232 MB heap size)
 PASS  __tests__/frontend-proxy.ts (253 MB heap size)
 PASS  __tests__/embed.ts (266 MB heap size)
 PASS  __tests__/redirects.ts (278 MB heap size)
 PASS  __tests__/legal-pages.tsx (297 MB heap size)
 PASS  __tests__/utils.tsx (304 MB heap size)
 PASS  __tests__/maintenance.ts (322 MB heap size)
 PASS  __tests__/lenabi.ts (328 MB heap size)
 PASS  __tests__/metadata-api.ts (340 MB heap size)
 PASS  __tests__/sentry.ts (357 MB heap size)
 PASS  __tests__/api.ts (370 MB heap size)
 PASS  __tests__/index.ts (387 MB heap size)
 PASS  __tests__/stats.ts (389 MB heap size)
 PASS  __tests__/packages.ts (401 MB heap size)
 PASS  __tests__/assets.ts (413 MB heap size)
 PASS  __tests__/pdf-proxy.ts (430 MB heap size)
 PASS  __tests__/quickbar-proxy.ts (442 MB heap size)

[kulla]

See also serlo/backlog#204