Get rid of fastTest property in TestSystemGroup.

It’s added to WikiPageDecorator to create specific test system
names for in process and out process runs.
Remove dependency on FitNesseContext for FitTestSystem.
Move test system creation to separate factory. Clean up TestSystemFactory interface.

This commit is a little big:
 * Remove Listener interface from TestSystemFactory.
 * Remove TestSystemGroup: test suites are executed in a linear fashion.
Inject TestSystemFactory in MultipleTestRunner.
Updated unit tests for MultipleTestsRunner.

Looks like the responsibilities changed somewhat.
Explicitly set TestingTracker on MultipleTestsRunner.

Still need to get rid of the FitNesseContext dependency.
Move page-by-test-system ordering to a separate class.

Executing tests and grouping tests are two separate concerns.
Little cleanup in ClassPathBuilder.
Externalise creation of Descriptor outside of PagesByTestSystem algorithm.
Let PagesByTestSystem and the surrounder class work with just wiki pages.

No need to wrap as TestPage already.
Make page order resolving logic somewhat more functional.

Do less (mis)use of state variables.
Merge branch 'master' into HEAD
Can load test systems via plugins loader.
Generalised common key:className code.
Hook up test systems loading to main app.
Remove unused method.
Merge branch 'master' into add-test-systems
Little cleanup in FitNesseMain.
Add acceptance test for adding custom test systems.
Replace constant by variable in SetUp.