GD-598: Fixing memory leaks on test execution

[MikeSchulze]

Why

Running tests via CMD line shows a lot of memory leaks at program exit.

What

• fixed releasing of all singletons
• func_assert has problems with lambdas when using inside timer.timeout.connect, converted lambda into function
• Handling of stored asserts in the thread context results in inconsistency during cleanup
• fixed cleanup on GdUnitCommandHandlerTest

Finally, I have reduced most of the memory leaks nodes, but there are still GDScriptFunctionState orphaned nodes that I can't solve because these are handled internally by Godot see 74449

Statistics: | 947 tests cases | 0 error | 0 failed | 1 flaky | 13 skipped | 0 orphans |

Executed test suites: (100/104), 4 skipped
Executed test cases: (934/947), 13 skipped
Total time:        3min 0s 409ms
Open Report at: file:///home/runner/work/gdUnit4/gdUnit4/reports/report_1/index.html
Exit code: 0
----------------------------------------------------------------
Cleanup singletons ["GdUnitThreadManager", "GdUnitDefaultValueDecoders", "GdUnitCommandHandler"]

	Unregister singleton 'GdUnitThreadManager'
	Free singleton instance 'GdUnitThreadManager:<Object#507678557642>'
	Successfully freed 'GdUnitThreadManager'

	Unregister singleton 'GdUnitDefaultValueDecoders'
	Free singleton instance 'GdUnitDefaultValueDecoders:<Object#509540832395>'
	Successfully freed 'GdUnitDefaultValueDecoders'

	Unregister singleton 'GdUnitCommandHandler'
	Free singleton instance 'GdUnitCommandHandler:<Object#3858659025580>'
	Successfully freed 'GdUnitCommandHandler'
----------------------------------------------------------------
Finallize .. done
Finallize ..
-Orphan nodes report-----------------------
Finallize .. done
XR: Clearing primary interface
XR: Removed interface "Native mobile"
XR: Removed interface "OpenXR"
WARNING: ObjectDB instances leaked at exit (run with --verbose for details).
     at: cleanup (core/object/object.cpp:2327)
Leaked instance: GDScriptFunctionState:9223373291404658414
Leaked instance: GDScriptFunctionState:92233732947[6010](https://github.com/MikeSchulze/gdUnit4/actions/runs/11956449331/job/33331231410?pr=599#step:4:6018)1646
Leaked instance: GDScriptFunctionState:9223373292545509197
Leaked instance: GDScriptFunctionState:9223373291673093970
Leaked instance: GDScriptFunctionState:9223373293719914330
Leaked instance: GDScriptFunctionState:9223373292881053539
Leaked instance: GDScriptFunctionState:9223373296521709422
Leaked instance: GDScriptFunctionState:9223378085393468557
Leaked instance: GDScriptFunctionState:9223382060754801221
Leaked instance: GDScriptFunctionState:9223373293753472331
Leaked instance: GDScriptFunctionState:9223377142497156399
Leaked instance: GDScriptFunctionState:9223378085359924447
Leaked instance: GDScriptFunctionState:9223382060788375283
Hint: Leaked instances typically happen when nodes are removed from the scene tree (with `remove_child()`) but not freed (with `free()` or `queue_free()`).
Orphan StringName: test_case1 (static: 0, total: 1)
Orphan StringName: _validate_callback (static: 0, total: 1)
Orphan StringName: await_millis (static: 0, total: 5)
Orphan StringName: test_timeout_single_yield_wait (static: 0, total: 1)
Orphan StringName: test_timeout_2s (static: 0, total: 1)
Orphan StringName: _execute (static: 0, total: 1)
Orphan StringName: test_timeout_4s (static: 0, total: 1)
Orphan StringName: timeout (static: 2, total: 8)
Orphan StringName: cb_is_equal (static: 0, total: 2)
Orphan StringName: test_timeout_long_running_test_abort (static: 0, total: 1)
Orphan StringName: test_timeout_and_assert_fails (static: 0, total: 1)
StringName: 11 unclaimed string names at exit.
Run tests ends with 0