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 <sam@samwhited.com>
Reviewed-by: Dominik Honnef <dominik@honnef.co>

secure/precis/enforce9.0.0_test.go

diff --git a/secure/precis/enforce9.0.0_test.go b/secure/precis/enforce9.0.0_test.go
index ed5759c..2097455 100644
--- a/secure/precis/enforce9.0.0_test.go
+++ b/secure/precis/enforce9.0.0_test.go
@@ -7,6 +7,9 @@
 package precis
 
 import (
+	"strings"
+
+	"golang.org/x/text/runes"
 	"golang.org/x/text/secure/bidirule"
 )