DEV: Fix flaky test in spec/lib/svg_sprite/svg_sprite_spec.rb (#35673)

We should clean up at the end of each test instead of at the start to
ensure that there aren't any side effects.

In this particular case, `SvgSprite.expire_cache` calls
`DistributedCache#clear` but the cache is only cleared at the end of a
transaction. If we clear the cache in a `before` hook, the cache isn't
actually cleared until the existing transaction for the test has ended.

spec/lib/svg_sprite/svg_sprite_spec.rb

@@ -3,10 +3,11 @@
 RSpec.describe SvgSprite do
   fab!(:theme)
 
-  before do
+  before { allow(Rails.env).to receive(:test?).and_return(false) }
+
+  after do
     SvgSprite.clear_plugin_svg_sprite_cache!
     SvgSprite.expire_cache
-    allow(Rails.env).to receive(:test?).and_return(false)
   end
 
   it "can generate a bundle" do