Bug 1887029 - Simplify TabAttributes to explicitly specify supported attributes, a=dmeehan

The only tab attribute which is ever persisted by SessionStore is
"customizemode". This patch limits the logic to only allow persisting and
restoring this attribute.

The browser_attributes.js test is also updated to use the "customizemode"
attribute for testing, rather than a custom specified attribute.

Minor test fix for ESR115.

Differential Revision: https://phabricator.services.mozilla.com/D208655