main: add test for error coming from the optimizer

This is just one optimizer pass that's easy to test for. There are
others, but I'm ignoring them for now.

The one other that would be reasonable to test is when starting a
goroutine with -gc=none, but I'm leaving that one for another time.

errors_test.go

@@ -23,6 +23,7 @@ func TestErrors(t *testing.T) {
 		"loader-invaliddep",
 		"loader-invalidpackage",
 		"loader-nopackage",
+		"optimizer",
 		"syntax",
 		"types",
 	} {

testdata/errors/optimizer.go

@@ -0,0 +1,18 @@
+package main
+
+import "runtime/interrupt"
+
+var num = 5
+
+func main() {
+	// Error coming from LowerInterrupts.
+	interrupt.New(num, func(interrupt.Interrupt) {
+	})
+
+	// 2nd error
+	interrupt.New(num, func(interrupt.Interrupt) {
+	})
+}
+
+// ERROR: optimizer.go:9:15: interrupt ID is not a constant
+// ERROR: optimizer.go:13:15: interrupt ID is not a constant