Improve resilience of TestPullFromCentralRegistryImplicitRefParts

Sometimes transient network issues will cause
TestPullFromCentralRegistryImplicitRefParts to end up pulling with the
v1 protocol. This violates the assumptions behind the test. To make the
test more robust, allow a few retries if any pull ends up using the v1
protocol.

Fixes #17214

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