validation: add more signals to killsig test

In `validation/killsig.t`, Test not only SIGTERM, but also SIGUSR1 as
well as SIGUSR2.

Signed-off-by: Dongsu Park <dongsu@kinvolk.io>

Author: Dongsu Park

validation/killsig.go

@@ -15,6 +15,8 @@ import (
 
 var signals = []string{
 	"TERM",
+	"USR1",
+	"USR2",
 }
 
 func main() {