test/zdtm/lib/Makefile: fix

When optimizing our Travis runs, I noticed that test/zdtm/lib is built 3
times in a row -- first time the binaries, 2nd and 3rd time the deps,
in parallel (sic!).

I took a look at Makefile and found this, and some more problems,
like:

 - dependencies are generated but not included
 - cleandep does not depend on clean
 - groups.c dependencies are not generated (or used)
 - "make all" does not make groups binary
 - overcomplicated and unused groups.{pid,out,cleanout} targets
 - extra -f flag to rm
 - etc.

This patch (hopefully) fixes all of the above. In addition, it provides
a few enhancements:
 - enables "silent make" rules, so now lib/ is in line with the rest of zdtm
 - adds "dep" target (not used, just for convenience)

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>