[TEST] allow to fully disable REST tests included parsing via -Dtests.rest=false

We currently look for REST tests on file system although they are disabled. We should not do that and move the check earlier on. This way third parties using our test infra, which don't have REST tests on file system, can effectively disable the REST tests, otherwise they would get initialization error despite having disabled them.  The downside is that the number of tests visualized is going to be zero instead of the real number of parsed REST tests, but there is nothing we can do about this. Tests get ignored anyways.