RFR: JDK-8065673: Makefile enable JAXP tests running

[Tristan Yan]

Hi everyone

Could you please to review this small makefile change for JAXP tests

http://cr.openjdk.java.net/~tyan/JDK-8065673/webrev.00/ <http://cr.openjdk.java.net/~tyan/JDK-8065673/webrev.00/>
http://cr.openjdk.java.net/~tyan/JDK-8065673/jaxp/webrev.00/ <http://cr.openjdk.java.net/~tyan/JDK-8065673/jaxp/webrev.00/>

corresponding bug is https://bugs.openjdk.java.net/browse/JDK-8065673 <https://bugs.openjdk.java.net/browse/JDK-8065673>

Here is the background for this change.
As you may know we’ve pushed some new jaxp tests into openjdk. Since these tests have been put into different repo with current idk tests. We need a makefile to support these tests running in jprt system and nightly. In the future, we may put these tests back to idk repo, we have to maintain a consistent makefile with current idk tests. So I steal the makefile from idk test repo and make minimal change as possible. 
For supporting running tests in jprt system one new target(jaxp_test) has been added in top level makefile and jaxp_test will be considered as part of core testset.

Thank you very much.
Tristan