Experiment with a way to test the bindings.

[nical]

The main idea is to write some tests in the rust code using the godot_test macro which generates functions that are only exported if the "gd_test" feature flag is enabled, and have a testing gdnative library call them. A simple godot app loads the library, calls run_tests and exits.

To try it out (on linux):

cd test;
cargo build
cp ../target/debug/libgdnative_test.so ./project/lib
godot --path ./project