Follow-up: Refactor the rest of add_license_form_spec.js to use shallow mount

In ee/spec/frontend/vue_shared/license_compliance/components/add_license_form_spec.js we had been using the old mountComponent utility function for unit tests.

This made testing feature flags a bit more difficult. We converted a few unit tests to use shallowMount in !34036 (merged) but due to MVC - minimail viable change we did not change all of them.

Related Disscussion !34036 (comment 372636717)

Implementation Plan

• In ee/spec/frontend/vue_shared/license_compliance/components/add_license_form_spec.js convert the usages of mountComponent to shallowMount

• Update selectors to use the selectores off of wrapper returned from shallowMount