Fix TabBar initialization issue and add tests

[kitbdev]

• fixes TabBar current tab not set correctly from previous version #97253

Fixed TabContainer emitting multiple signals when being added to the tree if a child was made visible (for the children visibility before entering tree test).

Adds tests for TabBar and TabContainer.
Includes basic tests for adding and removing tabs, setting current tab, adding tabs when out of tree (important for initialization), and some layout tests.
This should help prevent regressions.
If there is too much, I can separate it into another PR.

TabBar and TabContainer both have a similar system to allow setting the current_tab before adding tabs, it might be possible to combine them in the future.

I had to make a public set_tab_offset since there was no other way to set the tab offset other than sending a mouse click to the offset buttons, ensure_tab_visible doesn't offer enough control. We should consider exposing it in the future.

Also added a debug message for if SIGNAL_CHECK_FALSE fails, it now says what the actual arguments are, just like SIGNAL_CHECK.

[YeldhamDev]

LGTM.

[akien-mga]

Thanks!