Add UI tests for security level
Some things to test:
- The toolbar button's tooltip and icon shows the current security level.
- If the toolbar button is removed (through customization) it is still configured at startup such that it works when added back to the toolbar.
- The security level panel shows the current security level and custom state.
- If the toolbar button is in the overflow menu (because the window width has become narrow), then activating the toolbar button will close the overflow menu and open the security level panel in its place.
- When the panel is shown, focus should be on the "Restore Defaults" button if we have a custom security level, or the setting button otherwise.
- The "Learn more" link should open the manual in a new tab in the same window.
- If the popup is shown by pressing "Enter" or "Space" on the toolbar button, and then closed with "Esc", then focus returns to the toolbar button. Similarly, when opening through the overflow menu, focus should return to the overflow menu button instead.
- If the "Restore Deafults" button is clicked the panel remains open, the custom messages is replaced with a description of the security level, the "Restore Defaults" button is hidden and the settings button receives focus instead.
- When the settings button is clicked about:preferences is opened, the setting is scrolled into view and focus is at the top of the group.
- If we have custom settings, there is a description in about:preferences and an option to restore them.
- Selecting another radio option in the preferences changes the security level.
- A change in the security level will change the underlying security level settings in the expected way.
Some of these relate to changes in #32274 (closed).