add an assertion util that looks for error logs related to "gemoc"

In our test suite, we are currently mostly doing system tests (based on swtbot)
however when some actions fails we do not have any relevant feedback in jenkins about the real cause of the failure. (because it fails in a previous action and jenkins did not capture the stdio of this step)
It would be useful to have a helper method that would look into the error log and search for messages involving gemoc component
it can be:
- the plugin throwing the error
- a stack trace involving an org.eclipse.gemoc.* class

ideally, it should also propose a way to ignore messages prior to a given time (or we need a simple way to remove older messages)