Add test:arel to test separately from adapters

[skipkayhil]

Previously, all of the Arel tests would be run with every database adapter. This is not necessarily a problem, but these tests end up running redundantly for each adapter/database combination even though they do not interact with adapters at all.

This commit follows up a commit in Rails [1] that added a new test:arel task for Active Record. This additional step creates a place for Arel to be tested a single time, so that a followup PR to Rails can filter out Arel tests when testing adapters.

1: rails/rails@f362f0796f2dd750fc03892ed45a57d145426bff