LU-13268 tests: customize lnet-selftest for performance

LU-13268 tests: customize lnet-selftest for performance

Sometimes we need to run read/write/ping tests separately
with different lst check flags.

Patch allows to:
  set the required list of tests via new lst_TESTS variable;
  set lst check to LST_BRW_CHECK_NONE, LST_BRW_CHECK_FULL
      or LST_BRW_CHECK_SIMPLE.

Test-Parameters: trivial testlist=lnet-selftest
Cray-bug-id: LUS-8005
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Change-Id: I502028f645f39f391c19b2028fc6ba5b7bdb1a96
Reviewed-on: https://review.whamcloud.com/37639
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>