Silence errors about invalid deferred callables during tests

[RandomShaper]

Addresses the complaint in #78987 (comment):

So we also get a lot of "ERROR: Trying to execute a deferred call/notification/set on a previously freed instance. Consider using queue_free() instead of free()." from running tests (you can see the CI run for this very PR). How should we avoid that?

NOTE: I'm not very fond of having to tweak the build system to let the TESTS_ENABLED macro reach the message queue implementation. Maybe the queue should expose some way of disabling the message. Or maybe even the error should only be printed when the debugger is attached so both the editor and tests are already out of the equation? Suggestions welcome.

[YuriSizov]

Can we actually identify the issues and fix them instead of trying to get progressively more clever about silencing the warning? What is the cause of the issue in general, and how do we find the source of it in tests, in PM, in editor?

[RandomShaper]

In most (all?) cases it's CanvasItem::_redraw_callback about Controls already deleted. At first sight, I can't think of an easy fix.

[reduz]

I am very unhappy with this pattern repeating

1. Code is broken in a PR because of a misunderstanding of how it works
2. Instead of understanding why it breaks, try to band-aid it.

IMO this should be closed and the original PR reverted and a lot of care needs to be taken when dealing with core.
Explanation here: #78987 (comment)