test: add ability to execute tests in parallel

All tests which executed in pid namespaces can be executed in parallel.

$ make zdtm_ns
real	0m54.806s
user	0m2.779s
sys	0m4.338s

$ make -j zdtm_ns
real	0m11.587s
user	0m3.361s
sys	0m5.134s

Another feature of this patch is that the subset of parallel tests is
executed before the subset of sequential tests.

If a first subset are not failed, the second one has a small chance to
fail, so a most part of bugs will be found in a few first seconds.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>