RFR JDK-8245842: provide tests for binary compatibility assertions for sealed classes

[Vicente Romero]

Hi,

Please review fix for [1] at [2], this patch provides test coverage for 
Chapter 13 Binary Compatibility of the Sealed Classes spec [3].

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8245842
[2] http://cr.openjdk.java.net/~vromero/8245842/webrev.00/
[3] 
http://cr.openjdk.java.net/~gbierman/jep360/jep360-20200526/specs/sealed-classes-jls.html