Site Editor: drill-down menu 'Back' button focus color and missing tooltips

[afercia]

Description

In the Site Editor, the'drill-down' on the left has now a 'Back' button that shows on all the levels.

• Visually, the button contains only an icon. As such, at the very least it should expose its accessible name in a tooltip. This is the standard for all the icon-only buttons.
• Ideally, the button accessible name should not change depending on the level, as the button visuals doesn't change.
• When the button is focused, the arrow color becomes dark blue and it's barely visible. This appears to happen only on the first level.

First level:

• The button icon becomes dark blue 😞
• The button does show a tooltip with text 'Dashboard' 👍
• Under the hood, the button accessible name doesn't match the visible tooltip text and it's Navigate to the Dashboard 😞 This is a barrier for speech recognition software users.

Second level:

• The button icon stays white 👍
• The button does not show a tooltip 😞
• Under the hood, the button accessible name changed to 'Back' 😞 (note: 'Back' is a good, short name. The point here is that it dynamically changed from the previous 'Navigate to the Dashboard' to 'Back'). This isn't great for speech recognition software users.

Third level:
Same as the second level.

Step-by-step reproduction instructions

• Go to the Site Editor.
• Navigate through the levels in the 'drill-down' menu.
• Check the icon color and inspect the source to confirm the issues described above.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[t-hamano]

First level:
The button icon becomes dark blue 😞

This point will be resolved by #48134. However, the tests fail, probably due to problems with the performance tests themselves, so they have not yet been merged.