RFR(XS): 8162670: make of jtreg_tests fails if no tests are run, causing jprt test runs to also fail

[Chris Plummer]

Hello,

Please review this simple change:

https://bugs.openjdk.java.net/browse/JDK-8162670
http://cr.openjdk.java.net/~cjplummer/8162670/webrev-00/

Note the copyright dates haven't been updated in this webrev, but I did 
update them locally after noticing that.

Tested with jprt test case given in the CR, and also with a jprt run 
using "testset -hotspot" to make sure I didn't break anything.

thanks,

Chris