Parallel execution only happens within TestRunner

[Kohsuke Kawaguchi]

While looking at the TestNG code, I noticed that the way parallel
execution is not optimal.

Currently, the parallel execution is used only per XmlTest. That is, if
you have 5 XmlTests, those 5 XmlTests will be executed sequentially,
and only XmlClasses within XmlTests are executed in parallel.

I think it'd be better if multiple XmlTests can be executed in
parallel. This is the natural expectation given that TestNG allows
parallel="true" to be placed on <suite>. It also improves the thread
utilization and reduces "join" operation, as XmlTests are more isolated
with each other than XmlClasses.

Also, when the goal of the parallel execution is to improve the
turn-around time on multi-processor systems, it usually doesn't make
much sense to specify # of threads in the testng.xml, since the ideal #
of threads are different depending on the system that it runs on.
Therefore, I wonder if it's better to let the command line or Ant task
specify (or at least override) the number of threads.