Fix testDate to identify invalid date and leap year

[douglas-chanwh]

Motivation:
This pull request is to fix the issue "Improve validators to ensure more TCK test coverage #81"

Explain here the context, and why you're making that change, what is the problem you're trying to solve.
I made use of the DateTimeFormatter.ISO_DATE.parse(value); to verify if the input date string is valid or not.
It can detect input strings with invalid values such as 32 October, 29 Feb 2021.

Conformance:

You should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md
Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines

[douglas-chanwh]

@vietj @InfoSec812 can you give me some comments on the PR? thanks

[pk-work]

@lindangcdrom Thanks for your help! Your change looks good to me. Let's wait until tomorrow in case someone has feedback. Otherwise I will merge it.

[InfoSec812]

@lindangcdrom I like all of the test cases you are covering, so that makes me pretty confident that this will be great!