test: move return-1605 regression test to _demo with validation

Move the regression test for issue #1608/#1604 from cl/_testgo to
_demo/go/return-1605 with proper validation logic that panics if
the returned slice has wrong length or elements.

Author: luoliwoshang

_demo/go/return-1605/main.go

@@ -0,0 +1,35 @@
+package main
+
+// Regression test for https://github.com/goplus/llgo/issues/1608
+
+type T struct {
+	data []int
+}
+
+func a() []int {
+	var t = T{data: []int{1, 2}}
+	a := t.data
+	t.data = []int{1, 2, 3}
+	return a
+}
+
+func b() ([]int, bool) {
+	var t = T{data: []int{1, 2}}
+	a := t.data
+	t.data = []int{1, 2, 3}
+	return a, true
+}
+
+func main() {
+	resultA := a()
+	if len(resultA) != 2 || resultA[0] != 1 || resultA[1] != 2 {
+		panic("a(): expect [1,2] but got different")
+	}
+
+	resultB, _ := b()
+	if len(resultB) != 2 || resultB[0] != 1 || resultB[1] != 2 {
+		panic("b(): expect [1,2] but got different")
+	}
+
+	println("ok")
+}