Add test that checks if custom callables don't crash when Err() is returned.

[Yarwin]

Before #944 returning Err() with custom callable was causing crash without any sensible backtrace (both on windows 10 and linux):

================================================================
handle_crash: Program crashed with signal 11
Engine version: Godot Engine v4.3.stable.official (77dcf97d82cbfe4e4615475fa52ca03da645dbd8)
Dumping the backtrace. Please include this when reporting the bug to the project developer.
[1] /lib/x86_64-linux-gnu/libc.so.6(+0x42520) [0x7f64ce442520] (??:0)
[2] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x410be5c] (??:0)
[3] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x410c4a9] (??:0)
[4] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x4318e70] (??:0)
[5] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232bafa] (??:0)
[6] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232ba0e] (??:0)
[7] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232ba0e] (??:0)
[8] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232ba0e] (??:0)
[9] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232ba0e] (??:0)
[10] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232ba0e] (??:0)
[11] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232ba0e] (??:0)
[12] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x232d0f8] (??:0)
[13] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x2347885] (??:0)
[14] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x4fe39a] (??:0)
[15] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x235e7fc] (??:0)
[16] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x23b64a5] (??:0)
[17] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x527ab4] (??:0)
[18] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x4202a2] (??:0)
[19] /lib/x86_64-linux-gnu/libc.so.6(+0x29d90) [0x7f64ce429d90] (??:0)
[20] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80) [0x7f64ce429e40] (??:0)
[21] /home/irwin/apps/godot/godot/G4/Godot_v4.3-stable_linux.x86_64() [0x43d44a] (??:0)
-- END OF BACKTRACE --
================================================================

This PR adds an extra test case that should shield against the regression in the future.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-950

[Bromeon]

Since that PR changed from by-value to by-ref, it might be a use-after-free: I could imagine that a value didn't live long enough, and now does because it's retained on the call-site.

So thanks a lot, a regression test is definitely a good idea in case we change implementation again 👍

[Bromeon]

Suggested change

	// errors in godot but should not crash
	assert_eq!(callable_with_err.callv(&varray![]), Variant::nil());
	// Errors in Godot, but should not crash.
	assert_eq!(callable_with_err.callv(&varray![]), Variant::nil());