Use Devise's swap mechanism in failure app test

Also refactor tests to remove the custom failure app class only used
once in favor of an inline class for the specific test, makes it easier
to follow what's going on.
Fix test that checks for proper configuration of omniauth callback

Previously the test was raising an ArgumentError by mistake:

    ArgumentError: wrong number of arguments (0 for 1)
        actionpack (4.1.4) lib/action_dispatch/routing/route_set.rb:328:in `eval_block'`

The #eval_block method expects a proc/lambda argument that can be
instance_exec'ed, not a real block. In the Rails implementation the block
is passed to #draw, which calls #eval_block internally passing the block
along, but as a Proc argument and not as a block.

Also the error we were raising from #devise_for was a RuntimeError,
changed to the expected ArgumentError. Adding an assertion on top of the
expected message should ensure we won't have this issue again.

This was introduced in 29da146c07fc4601f23fc610c916a630da4553b4, related
to #2802.
Use a different mapping name for testing the omniauth callback failure

This leaks into Devise mappings overriding the existing :admin one, so
depending on the test seed it fails like this:

    $ TESTOPTS="--seed=9972" rake

    ==> Devise.orm = :active_record
    Run options: --seed=9972

    ...

      1) Failure:
      MappingTest#test_allows_path_to_be_given [./test/mapping_test.rb:31]:
      Expected: "admin_area"
        Actual: "admin"

Using a different name should avoid any test randomization issues.