RFR: 8139271: Add top-level Makefile target to run hotspots jtreg tests

[Erik Helin]

Hi all,

this patch adds a new top-level Makefile to run the hotspot jtreg tests.
This is already possible today by running:
$ make test TEST="hotspot_all"

The new target, test-hotspot-jtreg, is a bit more discoverable in my
opinion (will also work with tab completion in supported shells).

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8139271

Webrev:
http://cr.openjdk.java.net/~ehelin/8139271/webrev.00/

Thanks,
Erik