Fix spec_helper.rb of web engine

This fixes the tests of the web engine when running the following from
its root: bundle exec rspec

An engine is meant to have some independence from the main application,
and its tests should be run independently.

This should be using the engine's own dummy application, actually, or
the dummy application of a dependency.

engines/web/spec/spec_helper.rb

@@ -1,8 +1,3 @@
-ENV["RAILS_ENV"] = "test"
-
-require File.expand_path("dummy/config/environment.rb", __dir__)
-require "rails/test_help"
-
-Rails.backtrace_cleaner.remove_silencers!
+require "../../spec/spec_helper.rb"
 
 Dir["#{File.dirname(__FILE__)}/support/**/*.rb"].each { |f| require f }