Fix elb tests

This fixes the ELB tests when run with FOG_MOCK=false: `FOG_MOCK=false bundle exec shindont tests/aws/models/elb`.

I'd appreciate any feedback on:

1. The way skipping of tests in incompatible environments is implemented
2. The location of the new `ec2_compatibility_mode` method
3. A more idiomatic way to teardown resources which may or may not actually exist