FIX: Cleanup DiscoursePluginRegistry state after tests that use it

This was causing some heisentests

spec/components/stylesheet/compiler_spec.rb

@@ -40,6 +40,7 @@ describe Stylesheet::Compiler do
       after do
         Discourse.plugins.pop
         Stylesheet::Importer.register_imports!
+        DiscoursePluginRegistry.reset!
       end
 
       it "does not include theme variables in plugins" do

spec/components/stylesheet/manager_spec.rb

@@ -68,7 +68,7 @@ describe Stylesheet::Manager do
 
   describe 'digest' do
     after do
-      DiscoursePluginRegistry.stylesheets.delete "fake_file"
+      DiscoursePluginRegistry.reset!
     end
 
     it 'can correctly account for plugins in digest' do