runtime: increase attempt count for map iteration order test.
runtime: increase attempt count for map iteration order test.

Some builders broke on this test; I'm guessing that was because
this test didn't try hard enough to find a different iteration order.

Update Issue 6719

R=dave
CC=golang-codereviews
https://codereview.appspot.com/47300043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang