Allow test cases to be run selectively

Allow test cases to be run selectively

When debugging a failing test with many test cases, it is useful
to be able to skip most tests. Introducing a new environment
variable VIR_TEST_RANGE=N-M enables execution of only the test
cases numbered N-M inclusive, starting from 1.

For example, to skip all the cgroup tests except 2

$ VIR_TEST_RANGE=2-3 VIR_TEST_DEBUG=1 ./vircgrouptest
TEST: vircgrouptest
 2) New cgroup for driver                                             ... Unexpected found LXC cgroup: 1
libvirt: Cgroup error : Failed to create controller cpu for group: No such file or directory
FAILED
 3) New cgroup for domain driver                                      ... Cannot find LXC cgroup: 1
libvirt: Cgroup error : Failed to create controller cpu for group: No such file or directory
FAILED

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>