ARC: runner: mdb: tweak searching for cld process pid

mdb binary starts several subproceses and one of them is cld process.
In runners/mdb.py we record process id of cld on each mdb launch
to terminate simulator correctly later. However we can finish test
and terminate mdb before the cld process was found (so cld won't
be terminated correctly by sanitycheck infrastructure). It may happen
if we launch mdb on fast host machine.

That leads to several issues. First of all we get ugly error in
sanitycheck output:
------------------------>8--------------------------------
FileNotFoundError: [Errno 2] No such file or directory: '/xxxx/mdb.pid'
------------------------>8--------------------------------

Secondly (and it's more important) we terminate simulator incorrectly.
We terminate mdb leaving cld process alive, running and consuming one
cpu core permanently (until we kill it manually)

So, let's increase granularity of lookups and don't wait extra 0.5
seconds before the first lookup.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>