Fix `test_copying_migrations_preserving_magic_comments`

Since #29540, `# frozen_string_literal: true` included original
migration files.

activerecord/lib/active_record/migration.rb

@@ -863,15 +863,17 @@ module ActiveRecord
         source_migrations.each do |migration|
           source = File.binread(migration.filename)
           inserted_comment = "# This migration comes from #{scope} (originally #{migration.version})\n"
-          if /\A#.*\b(?:en)?coding:\s*\S+/ =~ source
+          magic_comments = "".dup
+          loop do
             # If we have a magic comment in the original migration,
             # insert our comment after the first newline(end of the magic comment line)
             # so the magic keep working.
             # Note that magic comments must be at the first line(except sh-bang).
-            source[/\n/] = "\n#{inserted_comment}"
-          else
-            source = "#{inserted_comment}#{source}"
+            source.sub!(/\A(?:#.*\b(?:en)?coding:\s*\S+|#\s*frozen_string_literal:\s*(?:true|false)).*\n/) do |magic_comment|
+              magic_comments << magic_comment; ""
+            end || break
           end
+          source = "#{magic_comments}#{inserted_comment}#{source}"
 
           if duplicate = destination_migrations.detect { |m| m.name == migration.name }
             if options[:on_skip] && duplicate.scope != scope.to_s

activerecord/test/cases/migration_test.rb

@@ -1015,8 +1015,8 @@ class CopyMigrationsTest < ActiveRecord::TestCase
     assert File.exist?(@migrations_path + "/4_currencies_have_symbols.bukkits.rb")
     assert_equal [@migrations_path + "/4_currencies_have_symbols.bukkits.rb"], copied.map(&:filename)
 
-    expected = "# coding: ISO-8859-15\n# This migration comes from bukkits (originally 1)"
-    assert_equal expected, IO.readlines(@migrations_path + "/4_currencies_have_symbols.bukkits.rb")[0..1].join.chomp
+    expected = "# frozen_string_literal: true\n# coding: ISO-8859-15\n# This migration comes from bukkits (originally 1)"
+    assert_equal expected, IO.readlines(@migrations_path + "/4_currencies_have_symbols.bukkits.rb")[0..2].join.chomp
 
     files_count = Dir[@migrations_path + "/*.rb"].length
     copied = ActiveRecord::Migration.copy(@migrations_path, bukkits: MIGRATIONS_ROOT + "/magic")

activerecord/test/migrations/magic/1_currencies_have_symbols.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 # coding: ISO-8859-15
 
 class CurrenciesHaveSymbols < ActiveRecord::Migration::Current