LU-7082 test: fix synchronization of conf_sanity test_90

LU-7082 test: fix synchronization of conf_sanity test_90

Add some delays in check_max_mod_rpcs_in_flight() routine
to ensure background commands are launched before continuing
test execution.

Signed-off-by: Gregoire Pichon <gregoire.pichon@bull.net>
Change-Id: Ia1c943b7b58ecfe4f3fd80d6470a8ee2650789e7
Reviewed-on: http://review.whamcloud.com/16215
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>