Add rake test task to run tests over the 3 DBs we support

Rakefile

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "bundler/setup"
 
 APP_RAKEFILE = File.expand_path("test/dummy/Rakefile", __dir__)
@@ -6,3 +8,14 @@ load "rails/tasks/engine.rake"
 load "rails/tasks/statistics.rake"
 
 require "bundler/gem_tasks"
+
+def databases
+  %w[ mysql postgres sqlite ]
+end
+
+task :test do
+  databases.each do |database|
+    sh("TARGET_DB=#{database} bin/setup")
+    sh("TARGET_DB=#{database} bin/rails test")
+  end
+end