zdtm.py: Generator of groups of tests

Introduce yet another tests set called 'groups'. Each test
in this set is a list of existing zdtm tests that can be
started side-by-side in an ns flavor.

To 'create' such a test the zdtm.py group action is used,
which lists tests and semi-randomly groups them together.
The grouping possibility is checked by comparing the .desc
files of those -- desc-s should coincide. One exception is
test dependencies, these are just merged together.

After running the group action there appears groups/ dir
with tests each containing just the list of zdtm tests
that are in a group. The respective .desc file is also
generated and this one matches the .desc for tests inside.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>