Make astrolabe resilient to faulty specification test files

[prashantmital]

Consider a situation wherein there exists a typo in one of the test scenario files - say an expected field is missing. As written currently, the test runner will notice the error when it attempts to access the missing field, probably in astrolabe.spec_runner.AtlasTestCase.run. Attempting to access this non-existent field would raise an AttributeError which would be propagated back to the user and terminate the entire test run.

astrolabe should be robustified to deal with this kind of erroneous test file content. A faulty spec test should only cause that particular test to fail, but should not impede the remainder of the test run from proceeding as usual.

[prashantmital]

UPDATE: since we now run one-test-per-task a faulty test scenario file will no longer impede other tests (for which the files don't have any errors). Consequently, this is no longer a major concern and can be closed.