RFR: 8139272: Add configure variable to set concurrency for jtreg tests

[Erik Helin]

Hi all,

this patch adds a new configure variable: --with-test-jobs. The new
variable configures how many tests jobs we run concurrently (aka the
-concurrency flag to JTReg). Today --with-jobs is passed as the
-concurrency flag to JTReg which most likely is too big for most
systems with many cores.

For hotspot, the only "supported" configuration for running the jtreg
tests is with -concurrency:1. However, most machines will run the tests
successfully with more concurrency, but I used a default of 1. For the JDK
tests I kept --with-jobs as the default concurrency since I want to keep
the old behaviour.

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8139272

Webrev:
http://cr.openjdk.java.net/~ehelin/8139272/webrev.00/

Thanks,
Erik