LU-12579 tests: allow some margin in runtests

LU-12579 tests: allow some margin in runtests

Allow some margin in the space used by runtests for internal
log files for Lustre and the underlying filesystem.

Test-Parameters: trivial testlist=runtests,runtests,runtests
Test-Parameters: mdtcount=4 mdscount=2 testlist=runtests,runtests,runtests
Test-Parameters: fstype=zfs testlist=runtests,runtests,runtests
Test-Parameters: fstype=zfs mdtcount=4 mdscount=2 testlist=runtests,runtests
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I34b47a8436c5718be311698a3f6e6d7af7ea45ad
Reviewed-on: https://review.whamcloud.com/36011
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>