cases: skip tests if unicode.Version does not match UnicodeVersion

For golang/go#11811.
Fixes golang/go#11927.

Change-Id: Ie60c687ba93aaeb6c164413289f474be63e0224f
Reviewed-on: https://go-review.googlesource.com/13252
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/cases/context_test.go b/cases/context_test.go
index d6c012f..2b94972 100644
--- a/cases/context_test.go
+++ b/cases/context_test.go
@@ -63,6 +63,9 @@
 }
 
 func TestCaseProperties(t *testing.T) {
+	if unicode.Version != UnicodeVersion {
+		t.Skipf("UnicodeVersion=%s, but unicode.Version=%s", UnicodeVersion, unicode.Version)
+	}
 	assigned := rangetable.Assigned(UnicodeVersion)
 	for r := rune(0); r <= lastRuneForTesting; r++ {
 		if !unicode.In(r, assigned) || !unicode.In(unicode.SimpleFold(r), assigned) {
@@ -89,6 +92,9 @@
 }
 
 func TestMapping(t *testing.T) {
+	if unicode.Version != UnicodeVersion {
+		t.Skipf("UnicodeVersion=%s, but unicode.Version=%s", UnicodeVersion, unicode.Version)
+	}
 	assigned := rangetable.Assigned(UnicodeVersion)
 	apply := func(r rune, f func(c *context) bool) string {
 		c := contextFromRune(r)