RFR: JDK-8187642: The consolidated repo test makefile disables CONCURRENCY setting to jtreg

[Erik Joelsson]

When invoking the test/Makefile using the jtreg_tests target in the 
consolidated repo, the CONCURRENCY variable gets overridden with an 
empty value. This was introduced with my changes to handle the new test 
roots. I believe the proper fix is to surround the override of 
CONCURRENCY with a conditional.

Bug: https://bugs.openjdk.java.net/browse/JDK-8187642
Webrev: http://cr.openjdk.java.net/~erikj/8187642/webrev.01/

/Erik