test: add a test file for relevant obsmarkers and its usage The logic around obsmarkers "relevant" to a set of revs have a couple of test around in other places but no systematic testing. In addition, all the current testing focus on the exchange case (we looks at relevant markers for '::heads'). For bundles, we'll need something a bit different. We'll no longer have set of revision going down to the repository roots. So we'll have to test these cases too. In addition, stripping obsmarkers will introduce new logic around obsmarkers that will need testing too. So a new test file make sense here. We start with a simple tests, more advanced cases are coming in the next changesets. The extra testing catch a minor bug (later in the series).