client: use designated domains in tests (RFC2606)

Update domains used in tests to used domains that are designated for this
purpose as described in [RFC2606, section 3][1]

[1]: https://www.rfc-editor.org/rfc/rfc2606.html#section-3

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

client/client_test.go

@@ -11,11 +11,11 @@ import (
 )
 
 const (
-	validServerAddress   = "https://index.docker.io/v1"
+	validServerAddress   = "https://registry.example.com/v1"
 	validUsername        = "linus"
 	validServerAddress2  = "https://example.com:5002"
 	invalidServerAddress = "https://foobar.example.com"
-	missingCredsAddress  = "https://missing.docker.io/v1"
+	missingCredsAddress  = "https://missing.example.com/v1"
 )
 
 var errProgramExited = fmt.Errorf("exited 1")