Update dependency jest to v26

package.json

@@ -25,7 +25,7 @@
     "eslint": "6.8.0",
     "eslint-config-synacor": "3.0.5",
     "identity-obj-proxy": "3.0.0",
-    "jest": "25.1.0",
+    "jest": "26.4.2",
     "jest-preset-preact": "1.0.0",
     "per-env": "1.0.2",
     "preact-cli": "3.0.0-rc.7",