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>
