Improve tests involving push_bulk (#17508)

sidekiq-bulk's push_bulk can either accept arguments directly or run them
through a block.

Setting expectations on the result of evaluating the blocks allows testing
more code (the block itself) and the test is moved closer to the *interface*
of the tested code than its precise implementation.

spec/spec_helper.rb

@@ -57,3 +57,10 @@ end
 def json_str_to_hash(str)
   JSON.parse(str, symbolize_names: true)
 end
+
+def expect_push_bulk_to_match(klass, matcher)
+  expect(Sidekiq::Client).to receive(:push_bulk).with(hash_including({
+    "class" => klass,
+    "args" => matcher
+  }))
+end