Update Composer config in PHP module (#3829)

* Prevents installation of `phpunit/phpunit:^6` because the test cases in this project require `\PHPUnit_Framework_TestCase` which was removed in v6. Moves generated PHP classes for tests into autoload-dev so that they are not referenced in the dist autoload. Adds Composer scripts to run protoc with and without grpc.

* update composer test target to use the same commands as generate_php_test_proto in tests.sh