Toolbar: Remove call to _setOptions from inside _makeFixed

Fixes gh-6567
Closes gh-6686
Toolbar: Remove check that makes sure there is only one back button

This is a partial revert of 86b70c6510c759ebc8f474a992c8e23b2c33a1c7
Toolbar: Add fixed toolbar integration test suite

Test to make sure that, when navigating to a page with a fixed toolbar that has
addBackBtn set to true, only one such back button is created.