RFR: JDK-8074016 Add convenient way of adding custom test targets to hotspot's test makefile

[Ingemar Aberg]

Please review this small fix to hotspot's test makefile

The file hotspot/test/Makefile provides '-include $(ALT_MAKE)/Makefile' 
close to the top (before most variables are set).
Move that line to after the variables are defined so they are available 
to the custom makefile.

Bug: https://bugs.openjdk.java.net/browse/JDK-8074016

Webrev: http://cr.openjdk.java.net/~sla/iaberg/8074016/webrev.00/

/Ingemar