Add a test RP to test navigator.id.get

fixes #3507
Address @callahad's great suggestions:

* Remove maximum-scale from the meta viewport declaration in the head
* Remove forceIssuer and allowUnverified in the call to process_assertion
Merge pull request #3508 from mozilla/issue_3507_get_rp

Add a test RP to test navigator.id.get

Dan gave the r+, merging.