Make long title test a little less cryptic

It's a little confusing have a string multiplication inside an
interpolation.

spec/lib/whitehall_importer/create_file_attachment_revision_spec.rb

@@ -52,9 +52,13 @@
   end
 
   context "when the file attachment does not satisfy requirements" do
-    let(:too_long_title) { "#{'A' * Requirements::Form::FileAttachmentUploadChecker::TITLE_MAX_LENGTH}A" }
     let(:whitehall_file_attachment) { build(:whitehall_export_file_attachment, title: too_long_title) }
 
+    let(:too_long_title) do
+      length = Requirements::Form::FileAttachmentUploadChecker::TITLE_MAX_LENGTH + 1
+      SecureRandom.alphanumeric(length)
+    end
+
     let(:error_message) do
       I18n.t!("requirements.title.too_long.form_message",
               max_length: Requirements::Form::FileAttachmentUploadChecker::TITLE_MAX_LENGTH)