Disable var usage for the test/unit folder

This allows us to enforce that `var` is not used anymore in the unit
tests to modernize the code and prevent subtle bugs.

test/unit/.eslintrc

@@ -0,0 +1,10 @@
+{
+  "extends": [
+    "../.eslintrc"
+  ],
+
+  "rules": {
+    // ECMAScript 6
+    "no-var": "error",
+  },
+}