Avoid importing "testing" in externally-facing code

The "testing" package adds some flags in its init function, so utilities
that import distribution code may print a page of extra testing flags in
their help output.

This commit solves the issue by moving an import of "testing" in the
registry/storage/cache package to a new
registry/storage/cache/cachecheck package, which is only imported by
tests.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

docs/storage/cache/redis/redis_test.go

@@ -6,7 +6,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/docker/distribution/registry/storage/cache"
+	"github.com/docker/distribution/registry/storage/cache/cachecheck"
 	"github.com/garyburd/redigo/redis"
 )
 
@@ -47,5 +47,5 @@ func TestRedisBlobDescriptorCacheProvider(t *testing.T) {
 		t.Fatalf("unexpected error flushing redis db: %v", err)
 	}
 
-	cache.CheckBlobDescriptorCache(t, NewRedisBlobDescriptorCacheProvider(pool))
+	cachecheck.CheckBlobDescriptorCache(t, NewRedisBlobDescriptorCacheProvider(pool))
 }