cranelift: move wasmtests in cranelift-wasm

Move test data used by cranelift-wasm's tests in
the crate directory, to make the tests autonomous.

Fixes #2910

cranelift/wasm/wasmtests/nullref.wat

@@ -0,0 +1,11 @@
+(module
+	(func (result externref)
+		(ref.null extern)
+	)
+
+	(func (result externref)
+		(block (result externref)
+			(ref.null extern)
+		)
+	)
+)