Add different types of mocking to frontend testing guidelines

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We currently use

• Jasmine Spys
• rewire
• a handful of custom mocks (for example spec/javascripts/helpers/vuex_action_helper.js)

and will soon use

• Vue Test Utils (https://gitlab.com/gitlab-org/gitlab-ce/issues/41069)
• Jest Mocks (https://gitlab.com/gitlab-org/gitlab-ce/issues/52483)

We need to document when to use which type of mocking.