Upgrade prettier to v2

Prettier v2 has the following breaking changes:
* enforces spaces between `function` and params
* enforces trailing commas by default

tasks/tests/.eslintrc.js

@@ -1,5 +1,5 @@
 module.exports = {
   env: {
-    mocha: true
-  }
+    mocha: true,
+  },
 };