Adding a mechanism to override tests in engine plugins from within

the application, in a structured way.

In your engine tests, add the following line to the bottom of the
file:

  Engines::Testing.override_tests_from_app

This will check for files in the application's test directory, of the
name "test/{kind}/{plugin_name}/{test_name}, and load it to override
any tests. For example, if the test is in

  vendor/plugins/my_plugin/test/unit/stuff_test.rb

then the 'override_tests' method will load (if the file exists)

  test/unit/my_plugin/stuff_test.rb

Keeping these overrides namespaced by plugin should make it a bit
clearer what's going on.

lib/engines/testing.rb

@@ -88,4 +88,14 @@ def self.set_fixture_path
     ActiveSupport::TestCase.fixture_path = self.temporary_fixtures_directory
     $LOAD_PATH.unshift self.temporary_fixtures_directory
   end
+
+  # overridden test should be in test/{unit,functional,integration}/{plugin_name}/{test_name}
+  def self.override_tests_from_app
+    filename = caller.first.split(":").first
+    plugin_name = filename.split("/")[-4]
+    test_kind = filename.split("/")[-2]
+    override_file = File.expand_path(File.join(File.dirname(filename), "..", "..", "..", "..", "..", "test", 
+                                               test_kind, plugin_name, File.basename(filename)))
+    load(override_file) if File.exist?(override_file)
+  end
 end