LU-12807 tests: fix intermittent runtests failure

LU-12807 tests: fix intermittent runtests failure

Occasional runtests failures are seen in full testing on ldiskfs.
Increase the llog space limit to 72KB from 50KB due to seeing a
regular failures in the 52/64KB range.

Lustre-change: https://review.whamcloud.com/44614
Lustre-commit: 14d07b623731233a62a8acd021c8ccdcb2705371

Test-Parameters: trivial testlist=runtests
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I6e272fe9fec21a650110a42efe31a1dc48e35854
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Vikentsi Lapa <vlapa@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46463
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>