strings: do much less redundant testing in TestCompareStrings

strings: do much less redundant testing in TestCompareStrings

On the OpenBSD builder this reduces the test time from 213 seconds to
60 seconds, without loss of testing.

Not sure why the test is so much slower on OpenBSD, so not closing the
issues.

Updates #26155
Updates #26174

Change-Id: I13b58bbe3b209e591c308765077d2342943a3d2a
Reviewed-on: https://go-review.googlesource.com/121820
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>