testdata: add more test cases in testdata

Specifically test for one or multiple init functions.

testdata/init_multi.go

@@ -0,0 +1,13 @@
+package main
+
+func init() {
+	println("init1")
+}
+
+func init() {
+	println("init2")
+}
+
+func main() {
+	println("main")
+}