unicode/utf8: add test that RuneCount does zero allocations
See disccusion in CL 612955.
Change-Id: I2de582321648f1798929ffb80d2f087e41146ead
Reviewed-on: https://go-review.googlesource.com/c/go/+/613315
Commit-Queue: Ian Lance Taylor <iant@google.com> Reviewed-by: Tim King <taking@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
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