Update fmt_test.go

- add test for double-digit, three-digit and four-digit argument index using common slice object (via @verytable comment)

varfmt/fmt_test.go

@@ -9,6 +9,11 @@ import (
 )
 
 func TestFormat(t *testing.T) {
+	var interfaceSlice []interface{} = make([]interface{}, 1002)
+	interfaceSlice[10] = 1
+	interfaceSlice[100] = 1
+	interfaceSlice[1000] = 1
+
 	for _, tc := range []struct {
 		format string
 		args   []interface{}
@@ -45,9 +50,19 @@ func TestFormat(t *testing.T) {
 			result: "Hello, World",
 		},
 		{
-			format: "{10}",
-			args:	[]interface{}{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10},
-			result:	"10",
+			format: "check {10}",
+			args:   interfaceSlice[:11],
+			result: "check 1",
+		},
+		{
+			format: "check {100}",
+			args:   interfaceSlice[:101],
+			result: "check 1",
+		},
+		{
+			format: "check {1000}",
+			args:   interfaceSlice[:1001],
+			result: "check 1",
 		},
 	} {
 		t.Run(tc.result, func(t *testing.T) {