[8u40] 8047740: Add hotspot testset to jprt.properties

[Mikael Vidstedt]

Please review this backport of 8047740 from 9 to 8u-dev. The backport is 
almost the same as the original changeset - only the version (jdk9 vs 
jdk8u20) is different, along with the corresponding 
--with-update-version configure argument. Currently the update version 
is set to '20' which matches what the corresponding 
hotspot/make/jprt.properties file uses.

Bug: https://bugs.openjdk.java.net/browse/JDK-8047740
jdk9 changeset: http://hg.openjdk.java.net/jdk9/dev/rev/9f96a36ef77c

webrev (8udev): 
http://cr.openjdk.java.net/~mikael/webrevs/8047740-8udev/webrev.00/webrev/

Cheers,
Mikael