Fix redefined method warnings

[aidanharan]

Fix the redefined method warnings when running the test suite.

Examples:

/vagrant/activerecord-sqlserver-adapter/test/cases/coerced_tests.rb:2564: warning: method redefined; discarding old invalid_add_column_option_exception_message
/vagrant/rails/activerecord/test/cases/migration/invalid_options_test.rb:10: warning: previous definition of invalid_add_column_option_exception_message was here
/vagrant/activerecord-sqlserver-adapter/test/cases/coerced_tests.rb:2576: warning: method redefined; discarding old thread_encrypting_and_decrypting
/vagrant/rails/activerecord/test/cases/encryption/concurrency_test.rb:15: warning: previous definition of thread_encrypting_and_decrypting was here
/vagrant/activerecord-sqlserver-adapter/test/cases/coerced_tests.rb:2593: warning: method redefined; discarding old serializer
/vagrant/rails/activerecord/test/cases/message_pack_test.rb:80: warning: previous definition of serializer was here