Replace erroneous use of this in test callback

The surrounding function is an arrow function, meaning this refers to
the outer this, which does not have a skip method.

This fixes 2/4 of existing test failures:

- should serialize: two lines string
- should serialize: two lines display string

Author: apasel422

test/httpwg-tests.js

@@ -269,7 +269,7 @@ function makeSerializeTest(test) {
 
         if (test.can_fail) {
           // Optional failure
-          this.skip('can_fail was true');
+          t.skip('can_fail was true');
         } else {
           throw e;
         }