Exercise connected_to and connects_to methods

Since both methods are public API I think it makes sense to add these tests
in order to prevent any regression in the behavior of those methods after the 6.0 release.

Exercise `connected_to`
  - Ensure that the method raises with both `database` and `role` arguments
  - Ensure that the method raises without `database` and `role`

Exercise `connects_to`
  - Ensure that the method returns an array of established connections(as mentioned
    in the docs of the method)

Related to #34052

activerecord/lib/active_record/connection_handling.rb

@@ -107,7 +107,7 @@ module ActiveRecord
     #   end
     def connected_to(database: nil, role: nil, &blk)
       if database && role
-        raise ArgumentError, "connected_to can only accept a database or role argument, but not both arguments."
+        raise ArgumentError, "connected_to can only accept a `database` or a `role` argument, but not both arguments."
       elsif database
         if database.is_a?(Hash)
           role, database = database.first

activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb

@@ -153,6 +153,20 @@ module ActiveRecord
           ENV["RAILS_ENV"] = previous_env
         end
 
+        def test_switching_connections_with_database_and_role_raises
+          error = assert_raises(ArgumentError) do
+            ActiveRecord::Base.connected_to(database: :readonly, role: :writing) { }
+          end
+          assert_equal "connected_to can only accept a `database` or a `role` argument, but not both arguments.", error.message
+        end
+
+        def test_switching_connections_without_database_and_role_raises
+          error = assert_raises(ArgumentError) do
+            ActiveRecord::Base.connected_to { }
+          end
+          assert_equal "must provide a `database` or a `role`.", error.message
+        end
+
         def test_switching_connections_with_database_symbol
           previous_env, ENV["RAILS_ENV"] = ENV["RAILS_ENV"], "default_env"
 
@@ -207,6 +221,27 @@ module ActiveRecord
           ActiveRecord::Base.configurations = @prev_configs
           ActiveRecord::Base.establish_connection(:arunit)
         end
+
+        def test_connects_to_returns_array_of_established_connections
+          config = {
+            "development" => { "adapter" => "sqlite3", "database" => "db/primary.sqlite3" },
+            "development_readonly" => { "adapter" => "sqlite3", "database" => "db/readonly.sqlite3" }
+          }
+          @prev_configs, ActiveRecord::Base.configurations = ActiveRecord::Base.configurations, config
+
+          result = ActiveRecord::Base.connects_to database: { writing: :development, reading: :development_readonly }
+
+          assert_equal(
+            [
+              ActiveRecord::Base.connection_handlers[:writing].retrieve_connection_pool("primary"),
+              ActiveRecord::Base.connection_handlers[:reading].retrieve_connection_pool("primary")
+            ],
+            result
+          )
+        ensure
+          ActiveRecord::Base.configurations = @prev_configs
+          ActiveRecord::Base.establish_connection(:arunit)
+        end
       end
 
       def test_connection_pools