LPS-47604 Make junit's forking jvm configurable

Brian, you were saying because we run 6.2.x and master on the same test env, it will be a problem to use different jdks to run  tests separately after master moves to jdk7.

So here is a solution, we can make the junit's forking jvm configurable. Before we run the tests, we can generate a build.${user.name}.properties, base on the current testing branch, pointing junit.jvm to different version of jdks.

By default it is loading from env PATH.