test: make maplinear iterdelete test less flaky

iterdelete's run time varies; occasionally we get unlucky. To reduce spurious failures, average away some of the variation.

On my machine, 8 of 5000 runs (0.15%) failed before this CL. After this CL, there were no failures after 35,000 runs.

I confirmed that this adjusted test still fails before CL 141270043.

LGTM=khr
R=khr
CC=bradfitz, golang-codereviews
https://codereview.appspot.com/140610043