[Core] Disconnect one-shot signals before calling callbacks

This prevents infinite recursion with one-shot connections emitting
themselves

modules/gdscript/tests/scripts/runtime/features/emit_after_await.gd

@@ -0,0 +1,12 @@
+# https://github.com/godotengine/godot/issues/89439
+extends Node
+
+signal my_signal
+
+func async_func():
+	await my_signal
+	my_signal.emit()
+
+func test():
+	async_func()
+	my_signal.emit()