Qunit plugin rake tasks (#4985)

* Allow running specific plugin tests using ENV variables

* Add a `rake plugin:qunit` task to match the existing `rake plugin:spec` task

* Improve docker.rake to allow running specific plugin qunit tests

* Purge cache before and after qunit tests

* Stop module auto-loader trying to auto-load tests

* Use URL query parameters to pass config into Qunit, avoiding caching issues

* Oops, searchParams doesn’t work in phantomJS. Parse the URL manually.

* Escape ampersands before passing URL to phantomJS, otherwise multiple parameters go wrong

lib/tasks/qunit.rake

@@ -39,12 +39,12 @@ task "qunit:test", [:timeout] => :environment do |_, args|
 
     options = {}
 
-    %w{module filter}.each do |arg|
+    %w{module filter qunit_skip_core qunit_single_plugin}.each do |arg|
       options[arg] = ENV[arg.upcase] if ENV[arg.upcase].present?
     end
 
     if options.present?
-      cmd += "?#{options.to_query.gsub('+', '%20')}"
+      cmd += "?#{options.to_query.gsub('+', '%20').gsub("&", '\\\&')}"
     end
 
     if args[:timeout].present?