Add proper test

[touilleMan]

• The current test folder has been renamed example
• A new test folder has been created, it contains a simple project which has a GDScript file running asserts on the built extension, then closing the app

Now I'm not sure how we should run this in the CI:

• For iOS&Android an emulator is needed, this seems overkill to me so it should be good enough to ignore those platforms for the moment
• To run the test we must have a Godot binary, which requires 1) download the latest official build or 2) build it here

As long as Godot 4 is not released, there is frequent breakage on Godot master, so downloading the latest official build seems not viable (but on the other hand it seems the way to go when Godot 4 is released)
On the other hand, building Godot from the master seems overkill, so I'm not sure we want that.

Even if we don't add this test to Godot-CPP's, we should run it in the Godot CI where Godot-CPP is currently build.

PS: I'm not sure I'll have much time to continue work on this, so feel free to modify this PR (or even copy part of it without bothering with crediting me and create your own PR 😄 )

[groud]

We reviewed this PR in the GDextension review meeting. We overall agree with the idea of spliting tests and examples into two different folders. Here are some feedback:

• The example.cpp/.h files in test should probably be renamed to test.cpp/.h.
• The example is a bit too complex now, and should limit itself to one property, and a few representative example functions.

[dsnopek]

Superseded by #1101