Add proper tests with JDK9+ modules

[jbescos]

Is your feature request related to a problem? Please describe.
Many new issues are coming because of JDK9+ modules. In my opinion we don't have a proper tests to verify/reproduce issues. Normally I have to create my own separate project to test this.

Describe the solution you'd like
I would like to have separate maven modules for testing purposes. In mail we should have few of them, because some apps are using the jakarta-mail bundle, and others not.

These new maven modules should have a src/java/main/module-info.java with a module name not related to jakarta.mail or com.sun.mail. For example: com.app.example.

In src/java/test we should add some simple tests to make sure there are no module issues.