Add tests for MigrationGrader

[tansaku]

MigrationGrader is in the code base but not tested

[tansaku]

I set up a test of the new grader for doing a migration grading, but we get this error

#<Graders::AutoGrader::NoSuchGraderError: Can't find grading strategy for MigrationGrader>

which I've encountered before - which is related to previous issue I mentioned in #63

I can quick fix that by adding the following temporarily to lib/autograder.rb:44

    require_relative 'graders/migration_grader/migration_grader.rb'

but that leads to this error:

expected PutResultException, got #<ArgumentError: Unable to find submission file "submissions/506af89a6181960fc69f47f1fbc8d708/performance-assignment/2016-02-19-12-20-54"> with backtrace:
        # ./lib/graders/migration_grader/migration_grader.rb:19:in `initialize'

which makes me think that the migration grader is still operating exactly at the old grader. So we would need to change the operation of the migration grader to work with wherever the submissions are now being inserted ...

[tansaku]

note of course that setting up this way still falls foul of #70 ...

[tansaku]

So the existing MigrationGrader is looking for an archive file submitted by the student, however the new grader is providing a directory ...

[tansaku]

the old migration grader unpacks the archive with a TempArchiveFile class that's no longer in the repo ...

[zhangaaron]

Not too aware of the migration grader, this was a low priority grader that was not updated with the autograder rehauling last summer. We aren't using it in Professor Fox's offering of the class, are you aware of any interest in using it in the online class?

[tansaku]

hi @zhangaaron it's being used in the online class (and has been every cycle for the last 3 years), and one of your fellow TAs recently asked for access to add two more homeworks to the Berkeley SPOC - not sure if this was one of them ...