language: use multiple runs in TestBestMatchAlloc
language: use multiple runs in TestBestMatchAlloc

AllocsPerRun sets runtime.GOMAXPROCS to 1, but it doesn't prevent the runtime from descheduling a goroutine
and performing an allocation somewhere in the background.
This commit changes the test to use a number of runs large enough to average away the occasional noisy allocation.

Fixes golang/go#45809

Change-Id: Ibf904016d0c067740469c8e861079611440222a1
GitHub-Last-Rev: 1606b6969a4d462804dda8d98b9c4432241e87ea
GitHub-Pull-Request: golang/text#23
Reviewed-on: https://go-review.googlesource.com/c/text/+/321737
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/text