This is a shot in the dark, but you might verify that the plugins providing the model provider are listed as dependencies for the test plugin.  My experience has shown that the test environment only includes hard dependencies, so if you're relying on something that has a soft link (e.g. anything contributed through an extension point), you need to make sure the plugin making the contribution (e.g. implementing the extension point) is included in the dependencies list for your test plugin.  It probably works fine in your dev env because Eclipse is using the entire set of plugins available (i.e. target platform + workspace plugins).