run_tests: allow run tests in parallel

run_task.sh is getting slow. This patch is trying to make it faster by
running the tests concurrently.

We provide a new parameter "-j" for the run_tests.sh, which can be used
to specify how many run queues we want for the tests. Default queue
length is 1, which is the old behavior.

Quick test on my laptop (4 cores, 2 threads each) shows 3x speed boost:

   |-----------------+-----------|
   | command         | time used |
   |-----------------+-----------|
   | run_test.sh     | 75s       |
   | run_test.sh -j8 | 27s       |
   |-----------------+-----------|

Signed-off-by: Peter Xu <peterx@redhat.com>
[Moved code to run_tests.bash, make "wait -n" support optional if -j1. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>