Toolbar: Remove call to _setOptions from inside _makeFixed

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

This is a partial revert of 86b70c6510c759ebc8f474a992c8e23b2c33a1c7
Tests 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.
Adds tests for cc54a86f41454bf481efb96cf40961db0e9af534 and this commit.

Closes gh-7084
Re gh-6554