LU-15282 tests: relax test_51d thresholds somewhat

LU-15282 tests: relax test_51d thresholds somewhat

Added combinations for sanity.sh test_51d are failing some fraction
of the time.  Relax thresholds somewhat to avoid spurious failures,
while keeping added configs to detect major regressions.

Test-Parameters: trivial
Fixes: fd5c915eff ("LU-15282 tests: improve sanity test_51d coverage")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I2d28f77fa377a1eb96a00cf35827b9ebc5af806b
Reviewed-on: https://review.whamcloud.com/47978
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>