Test enhancement

[peter279k]

Changed log

• Add php-7.3 test in Circleci build because the php-7.3 version is stable release now.
• Add the different released PHPUnit versions to support multiple PHP versions.
• Add the autoload-dev block in composer.json and define the class namespaces in tests to load these classes automatically.
• Remove useless require_once code because the classes will be loaded automatically via autoload.php.
• The successful Circle CI build result is available here.
• Add the suggest block in composer.json to recommend the gmp and bcmath extensions.
  It's related to issue Add a word about php gmp requirement #11.

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[peter279k]

I signed it!

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