MRP-1101 tests: avoid stat timeout in lnet-selftest

When batched tests are running, stat rpcs timeout regularly due to
network congestion.  Timeouts which happen during 'lst stat' are
ignored.  Timeouts during 'lst show_error' called in lst_end_session
are considered as test failure. In order to avoid those timeouts -
stop batched tests before call to 'lst show_error'

Signed-off-by: Vladimir Saveliev <vladimir_saveliev@xyratex.com>
Change-Id: I42be74d74001937656509cbd6b265d9bbfc2d1fe