The ParseLink() function has special handling for legacy formats;
> This is kept because we can actually get a HostConfig with links
> from an already created container and the format is not `foo:bar`
> but `/foo:/c1/bar`
This patch adds a test-case for this format. While updating, also switching
to use gotest.tools assertions.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>