Consider upgrading PHPUnit version

[peter279k]

Changed log

• Firstly, it's about the enhancement work.
  According to the official PHPUnit website, the PHPUnit versions support different PHP versions. For example, looking at following items:
  • The PHPUnit 5.x supports php-5.6 version at least.
  • The PHPUnit 6.x supports php-7.0 version at least.
  • The PHPUnit 7.x supports php-7.1 version at least.
  • The PHPUnit 8.x supports php-7.2 version at least.

To let unit test works executing successful, I suggest we can consider upgrading the PHPUnit version and defining multiple PHPUnit versions to support different PHP versions.

And the unit test development on this package will be good and reliable.

BTW, once this issue is accepted, I can work on that :).

[bhelx]

This would be much appreciated! I'll happily review and merge this.