Squelch output of ActiveSupport::Cache::NullStore in tests

When a spec fails relating to the cache, the job log can easily exceed
the maximum length and make it harder to see the actual issue
(e.g. https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/239803017).

spec/support/inspect_squelch.rb

+# This class can generate a lot of output if it fails,
+# so squelch the instance variable output.
+class ActiveSupport::Cache::NullStore
+  def inspect
+    "<#{self.class}>"
+  end
+end