Not reseting the registry could lead to assets still being registered for example.
This flakky spec was reprdocible with this call: `bundle exec rspec --seed 9472 spec/components/discourse_plugin_registry_spec.rb spec/components/svg_sprite/svg_sprite_spec.rb`
Which would trigger the following error:
```
Failures:
1) DiscoursePluginRegistry#register_asset registers vendored_core_pretty_text properly
Failure/Error: expect(registry.javascripts.count).to eq(0)
expected: 0
got: 1
(compared using ==)
# ./spec/components/discourse_plugin_registry_spec.rb:248:in `block (3 levels) in <top (required)>'
# ./spec/rails_helper.rb:280:in `block (2 levels) in <top (required)>'
# /Users/joffreyjaffeux/.gem/ruby/2.7.3/gems/webmock-3.14.0/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'
```
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