Added rspec test for AssignmentFile

Conversion of test/unit/assignment_file_test.rb to RSpec

It runs well...

AssignmentFile
  a good AssignmentFile model
    should require case sensitive unique value for filename scoped to assignment_id
    should allow filename to be set to "est.java"
    should not allow filename to be set to "&*%word^!"
    should belong to assignment
    should not allow filename to be set to "est?java"
    should require filename to be set

@toriverly I think this test should remain.