RFR: JDK-8176084 Developer-friendly run-test facility

[Magnus Ihse Bursie]

A long-time issue has been a consistent way for developers to 
effortlessly run tests on local builds. This patch introduces a new, 
alternative "run-test" target, which allows for a smoother developer 
experience in running tests. It does not modify or remove any existing 
ways of running tests, which are still needed for automated test systems 
and old scripts.

Bug: https://bugs.openjdk.java.net/browse/JDK-8176084
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8176084-introduce-run-test/webrev.01

/Magnus