testutils: Move DontRecover flag

It was used only in queue tests and so was moved there. This fixes
-tests.debuglog command line flag being present in the maddy executable
as a side effect of testutils package import.

target/queue/queue_test.go

@@ -817,5 +817,5 @@ func TestQueueDSN_RcptRewrite(t *testing.T) {
 }
 
 func init() {
-	testutils.DontRecover = true
+	dontRecover = true
 }