testing: Init plugin config when for tests

This fixes a panic when running this test for me locally.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

testutil/fixtures/plugin/plugin.go

@@ -91,6 +91,7 @@ func CreateInRegistry(ctx context.Context, repo string, auth *types.AuthConfig,
 	}
 
 	var cfg Config
+	cfg.PluginConfig = &types.PluginConfig{}
 	for _, o := range opts {
 		o(&cfg)
 	}