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/memory/memory_test.go

@@ -3,11 +3,11 @@ package memory
 import (
 	"testing"
 
-	"github.com/docker/distribution/registry/storage/cache"
+	"github.com/docker/distribution/registry/storage/cache/cachecheck"
 )
 
 // TestInMemoryBlobInfoCache checks the in memory implementation is working
 // correctly.
 func TestInMemoryBlobInfoCache(t *testing.T) {
-	cache.CheckBlobDescriptorCache(t, NewInMemoryBlobDescriptorCacheProvider())
+	cachecheck.CheckBlobDescriptorCache(t, NewInMemoryBlobDescriptorCacheProvider())
 }