Engine hangs on exit after executing GDScript unit tests using GUT

[Xrayez]

Godot version:
3.2.4 beta 4162df4.
Related to #42305. (CC @RandomShaper).

OS/device including version:
Tested on Windows 10, Linux server platforms.

Issue description:
After #42323, the crash is resolved, but the engine hangs after all tests are finished. When I pause the debugger, occasionally this ends up with a call stack similar to this:

godot.windows.tools.64.goost.exe!Ref<class Script>::Ref<class Script>(void) (Unknown Source:0)
godot.windows.tools.64.goost.exe!GDScriptLanguage::~GDScriptLanguage() Line 2256 (d:\usr\src\godot\goostengine\goost\godot\modules\gdscript\gdscript.cpp:2256)
godot.windows.tools.64.goost.exe!GDScriptLanguage::`scalar deleting destructor'(unsigned int) (Unknown Source:0)
godot.windows.tools.64.goost.exe!memdelete<GDScriptLanguage>(GDScriptLanguage * p_class) Line 119 (d:\usr\src\godot\goostengine\goost\godot\core\os\memory.h:119)
godot.windows.tools.64.goost.exe!unregister_gdscript_types() Line 180 (d:\usr\src\godot\goostengine\goost\godot\modules\gdscript\register_types.cpp:180)
godot.windows.tools.64.goost.exe!unregister_module_types() Line 228 (d:\usr\src\godot\goostengine\goost\godot\modules\register_module_types.gen.cpp:228)
godot.windows.tools.64.goost.exe!Main::cleanup() Line 2255 (d:\usr\src\godot\goostengine\goost\godot\main\main.cpp:2255)
godot.windows.tools.64.goost.exe!widechar_main(int argc, wchar_t * * argv) Line 164 (d:\usr\src\godot\goostengine\goost\godot\platform\windows\godot_windows.cpp:164)
godot.windows.tools.64.goost.exe!_main() Line 184 (d:\usr\src\godot\goostengine\goost\godot\platform\windows\godot_windows.cpp:184)
godot.windows.tools.64.goost.exe!main(int _argc, char * * _argv) Line 196 (d:\usr\src\godot\goostengine\goost\godot\platform\windows\godot_windows.cpp:196)
[Inline Frame] godot.windows.tools.64.goost.exe!invoke_main() Line 78 (d:\agent\_work\4\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78)
godot.windows.tools.64.goost.exe!__scrt_common_main_seh() Line 288 (d:\agent\_work\4\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288)
kernel32.dll!00007ffe5d5e7bd4() (Unknown Source:0)
ntdll.dll!00007ffe5d9ace51() (Unknown Source:0)

Seems to happen on project get_tree().quit() once tests are finished.

See GUT log at https://github.com/goostengine/goost/runs/1164928377, where the testing stage kept running for 5 hours, until it got automatically cancelled by GitHub, which fail CI result.

Steps to reproduce:
Likely running some GUT tests could reproduce this, but in case if you need the exact state:

git clone https://github.com/goostengine/goost
cd goost
scons

Then run GUT tests using the tests/run.sh with the resulting binaries, or manually...

Minimal reproduction project:
I haven't yet created a minimal project to test this out, seems to happen with more complex testing procedures, but perhaps this could already provide some insights.