add: test timestamp extraction

Change-Id: I4aba2b7d387d8cb7b9c1baec997e59b1b594c34d

internal/scaffold/demo.go

@@ -48,11 +48,18 @@ var demoTestCase = &hrp.TestCase{
 			WithBody(map[string]interface{}{
 				"foo1": "$varFoo1",       // reference former extracted variable
 				"foo2": "${max($a, $b)}", // 12.3; step level variables are independent, variable b is 3.45 here
+				"time": "${get_timestamp()}",
 			}).
+			Extract().
+			WithJmesPath("body.form.time", "varTime").
 			Validate().
 			AssertEqual("status_code", 200, "check status code").
 			AssertLengthEqual("body.form.foo1", 5, "check args foo1").
 			AssertEqual("body.form.foo2", "12.3", "check args foo2"), // form data will be converted to string
+		hrp.NewStep("get with timestamp").
+			GET("/get").WithParams(map[string]interface{}{"time": "$varTime"}).
+			Validate().
+			AssertLengthEqual("body.args.time", 13, "check extracted var timestamp"),
 	},
 }