secure/precis: add missing strings, x/text/runes imports for 1.9
secure/precis: add missing strings, x/text/runes imports for 1.9

This is a followup to CL 113816. That CL added identical new code to
enforce10.0.0_test.go and enforce9.0.0_test.go, but didn't import new
packages in the latter file. This CL fixes that. As a result, package
tests now pass in Go 1.9.

Updates golang/go#24885.

Change-Id: I0286d873b7f3239bce1a151570c4f9c7ac0219c1
Reviewed-on: https://go-review.googlesource.com/121359
Reviewed-by: Sam Whited <f16bed56189e249fe4ca8ed10a1ecae60e8ceac0@samwhited.com>
Reviewed-by: Dominik Honnef <aeacd46bb4fd3f59504c7762d9af6bd04f20aa1f@honnef.co>

golang/text