Introduce `@Ignore`s in two-fer.

exercises/two-fer/HINT.md

@@ -0,0 +1,3 @@
+To help you focus on one test at a time, [@Ignore](http://junit.sourceforge.net/javadoc/org/junit/Ignore.html)
+annotations have been added to every test but the first one. Any test with an `@Ignore` annotation will be skipped
+when you run the tests.