Currently, the `tap package test` CLI action is rarely used. I suspect this is because it is not very flexible, and must be defined in terms of a shell command in the package.xml file. This is also not very cross-platform friendly. I think we should add an interface to OpenTAP, e.g. `IPackageTestAction`. OpenTAP could then invoke the tests specified by interface implementors in addition to any test actionsteps specified in package.xml when `tap package test MyPackage` is run. An example of how I imagine a plugin could implement tests: