Fix and re-enable Add-On UI tests to account for new delay in Add button state

Introduced in https://github.com/mozilla-mobile/reference-browser/commit/5f362e52713c0da329576e2bdd3825edf140802b `Add` has an internal count-down so the tests need to wait until it's enabled.

* `installAddonTest`
* `removeAddonTest`
* `verifyAddonElementsTest`

Same similar fix as we did in Fenix required (e.g, https://bugzilla.mozilla.org/show_bug.cgi?id=2044133)