DEV: Allow fab! without block (#24314)

The most common thing that we do with fab! is:

    fab!(:thing) { Fabricate(:thing) }

This commit adds a shorthand for this which is just simply:

    fab!(:thing)

i.e. If you omit the block, then, by default, you'll get a `Fabricate`d object using the fabricator of the same name.

spec/lib/file_store/local_store_spec.rb

@@ -5,11 +5,11 @@ require "file_store/local_store"
 RSpec.describe FileStore::LocalStore do
   let(:store) { FileStore::LocalStore.new }
 
-  fab!(:upload) { Fabricate(:upload) }
+  fab!(:upload)
   let(:uploaded_file) { file_from_fixtures("logo.png") }
   let(:upload_path) { Discourse.store.upload_path }
 
-  fab!(:optimized_image) { Fabricate(:optimized_image) }
+  fab!(:optimized_image)
 
   describe "#store_upload" do
     it "returns a relative url" do

spec/lib/file_store/s3_store_spec.rb

@@ -12,7 +12,7 @@ RSpec.describe FileStore::S3Store do
   let(:s3_object) { stub }
   let(:upload_path) { Discourse.store.upload_path }
 
-  fab!(:optimized_image) { Fabricate(:optimized_image) }
+  fab!(:optimized_image)
   let(:optimized_image_file) { file_from_fixtures("logo.png") }
   let(:uploaded_file) { file_from_fixtures("logo.png") }
   fab!(:upload) { Fabricate(:upload, sha1: Digest::SHA1.hexdigest("secret image string")) }