Define tap package test in code

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: