roman: add property based test with quick.check

roman/roman_test.go

@@ -3,6 +3,7 @@ package roman
 import (
 	"fmt"
 	"testing"
+	"testing/quick"
 )
 
 var cases = []struct {
@@ -65,3 +66,15 @@ func TestConvertingToArabic(t *testing.T) {
 		})
 	}
 }
+
+func TestPropertiesOfConversion(t *testing.T) {
+	assertion := func(arabic int) bool {
+		roman := ConvertToRoman(arabic)
+		fromRoman := ConvertToArabic(roman)
+		return fromRoman == arabic
+	}
+
+	if err := quick.Check(assertion, nil); err != nil {
+		t.Error("failed checks", err)
+	}
+}