Test files polluting AppData/Local directory

[KoBeWi]

Tested versions

4.3 dev f58a96c

System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 31.0.15.4633) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

Issue description

Some tests use ResourceSaver (and probably FileAccess) to write some test files. Example:

godot/tests/core/io/test_resource.h

Lines 79 to 82 in f58a96c

	const String save_path_binary = OS::get_singleton()->get_cache_path().path_join("resource.res");
	const String save_path_text = OS::get_singleton()->get_cache_path().path_join("resource.tres");
	ResourceSaver::save(resource, save_path_binary);
	ResourceSaver::save(resource, save_path_text);

The problem is that they save it without any subfolder, creating a mess in that directory. On Windows it's AppData/Local, which I doubt it's intended for such things.

There could be a helper method that returns a safe cache directory, which could be a sub-directory of the above folder (with ensured existence).

Steps to reproduce

1. Run tests (godot.exe --test)

Minimal reproduction project (MRP)

N/A

[huwpascoe]

Windows has a temporary files folder for this purpose and an alias to access it:
%Temp%\file.txt expands to C:\Users\user\AppData\Local\Temp\file.txt

More to the point, the tests should clean up after themselves!