Make testname and creation of TestInsertionMojo configurable.

* Instead of disabling test-injection by reconfiguring the plugin
  provide a property `maven-hpi-plugin.disabledTestInjection` which is easily overridden
  in plugins.
* By renaming the injected test to InjectIT the test is not run
  during phase test but e.g. via the failsafe-plugin during phase
  integration-test. This can by done by setting property `maven-hpi-plugin.injectedTestName`.