added commandline and stdin tests using testscript (#28)

* added commandline and stdin tests using testscript

---------

Co-authored-by: Thomas von Dein <tom@vondein.org>

main_test.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"os"
+	"testing"
+
+	"github.com/rogpeppe/go-internal/testscript"
+)
+
+func TestMain(m *testing.M) {
+	os.Exit(testscript.RunMain(m, map[string]func() int{
+		"testrpn": Main,
+	}))
+}
+
+func TestRpn(t *testing.T) {
+	testscript.Run(t, testscript.Params{
+		Dir: "t",
+	})
+}