subheader: tests for subheader are useless.

[devversion]

Currently the subheader tests are completely meaningless and useless.

There are several issues:

• We're not injecting our material.components.subheader module
• We're mocking the complete $mdSticky service.

So at the moment we're testing things like ng-if with the $mdSticky mock and without our actual directive, because it can't be loaded.

This is actually a test, which just tests the standard AngularJS directive.

I'm working on some improvements on those tests.