Add test:arel task to run only Arel tests

Previously, all of the Arel tests would be run with every database
adapter. This is not necessarily a problem, but these tests end up
getting run along with every adapter's tests in CI even though none of
them require a database.

This commit starts the process of testing Arel separately from database
adapters by adding a new task to run Arel's tests. The next step will
be to add the test:arel task to Buildkite, and then the Arel tests can
be filtered from the list of adapter tests so that they are only run
once.

activerecord/Rakefile

@@ -38,6 +38,14 @@ namespace :test do
       %w(isolated_test_mysql2 isolated_test_sqlite3 isolated_test_postgresql)
     run_without_aborting(*tasks)
   end
+
+  Rake::TestTask.new(:arel) do |t|
+    t.libs << "test"
+    t.test_files = FileList["test/cases/arel/**/*_test.rb"]
+
+    t.warning = true
+    t.verbose = true
+  end
 end
 
 namespace :db do