Improve descriptions for Performance/ActiveRecordSubtransactionMethods

Prior this commit the example description did not show the method name
we were testing:

    RuboCop::Cop::Performance::ActiveRecordSubtransactionMethods
      when the method uses a subtransaction
        behaves like a method that uses a subtransaction
          registers an offense
        behaves like a method that uses a subtransaction
          registers an offense
        behaves like a method that uses a subtransaction
          registers an offense
        behaves like a method that uses a subtransaction
          registers an offense
        behaves like a method that uses a subtransaction
          registers an offense
        behaves like a method that uses a subtransaction
          registers an offense

This commit makes the example description more approachable:

    RuboCop::Cop::Performance::ActiveRecordSubtransactionMethods
      when the method uses a subtransaction
        method: :safe_ensure_unique
          behaves like a method that uses a subtransaction
            registers an offense
        method: :safe_find_or_create_by
          behaves like a method that uses a subtransaction
            registers an offense
        method: :safe_find_or_create_by!
          behaves like a method that uses a subtransaction
            registers an offense
        method: :with_fast_read_statement_timeout
          behaves like a method that uses a subtransaction
            registers an offense
        method: :create_or_find_by
          behaves like a method that uses a subtransaction
            registers an offense
        method: :create_or_find_by!
          behaves like a method that uses a subtransaction
            registers an offense

Also, use variables in `expect_offense` improving readability.

spec/rubocop/cop/performance/active_record_subtransaction_methods_spec.rb

 # frozen_string_literal: true
 
 require 'fast_spec_helper'
+require 'rspec-parameterized'
+
 require_relative '../../../../rubocop/cop/performance/active_record_subtransaction_methods'
 
 RSpec.describe RuboCop::Cop::Performance::ActiveRecordSubtransactionMethods do
@@ -10,16 +12,18 @@ RSpec.describe RuboCop::Cop::Performance::ActiveRecordSubtransactionMethods do
 
   shared_examples 'a method that uses a subtransaction' do |method_name|
     it 'registers an offense' do
-      expect_offense(<<~RUBY)
-        Project.#{method_name}
-                #{'^' * method_name.length} #{message}
+      expect_offense(<<~RUBY, method_name: method_name, message: message)
+        Project.%{method_name}
+                ^{method_name} %{message}
       RUBY
     end
   end
 
   context 'when the method uses a subtransaction' do
-    described_class::DISALLOWED_METHODS.each do |method|
-      it_behaves_like 'a method that uses a subtransaction', method
+    where(:method) { described_class::DISALLOWED_METHODS.to_a }
+
+    with_them do
+      include_examples 'a method that uses a subtransaction', params[:method]
     end
   end
 end