(XS) RFR: Add sanity test for minimal VM in test/Makefile

[David Holmes]

https://bugs.openjdk.java.net/browse/JDK-8058919

http://cr.openjdk.java.net/~dholmes/8058919/webrev/

test/Makefile defines the simple sanity check tests: clienttest and 
servertest. To that we add minimaltest to test the minimal VM.

This will be used by our JPRT system to ensure there is no accidental 
breakage of the minimal VM.

Thanks,
David