Clear memoized manifest to solve intermittent spec errors

`spec/lib/gitlab/webpack/manifest_spec.rb` appears to be failing
intermittently due to a memoized manifest that is loaded before the
tests are run. We now clear these values between each run to ensure a
clean state.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/255176

lib/gitlab/webpack/manifest.rb

@@ -24,6 +24,8 @@ module Gitlab
       AssetMissingError = Class.new(StandardError)
 
       class << self
+        include Gitlab::Utils::StrongMemoize
+
         def entrypoint_paths(source)
           raise WebpackError, manifest["errors"] unless manifest_bundled?
 
@@ -55,6 +57,10 @@ module Gitlab
           end
         end
 
+        def clear_manifest!
+          clear_memoization(:manifest)
+        end
+
         private
 
         def manifest_bundled?
@@ -67,7 +73,7 @@ module Gitlab
             load_manifest
           else
             # ... otherwise cache at class level, as JSON loading/parsing can be expensive
-            @manifest ||= load_manifest
+            strong_memoize(:manifest) { load_manifest }
           end
         end
 

spec/lib/gitlab/webpack/manifest_spec.rb

@@ -16,6 +16,14 @@ RSpec.describe Gitlab::Webpack::Manifest do
     EOF
   end
 
+  around do |example|
+    Gitlab::Webpack::Manifest.clear_manifest!
+
+    example.run
+
+    Gitlab::Webpack::Manifest.clear_manifest!
+  end
+
   shared_examples_for "a valid manifest" do
     it "returns single entry asset paths from the manifest" do
       expect(Gitlab::Webpack::Manifest.asset_paths("entry2")).to eq(["/public_path/entry2.js"])