Provide one true way to run the test suite

[mfn]

I checked out the project and it wasn't clear to me how to run the tests (as its not using phpunit). After searching around I found two variations of how to run the tests.

Adding a scripts entry to composer provides a single definition on how to run the tests, usable for every one; also locally:

$ composer tests
> tester -p php tests
 _____ ___  ___ _____ ___  ___
|_   _/ __)( __/_   _/ __)| _ )
  |_| \___ /___) |_| \___ |_|_\  v2.3.2

Note: No php.ini is used.
PHP 7.4.7 (cli) | php -n | 8 threads

.........................


OK (25 tests, 1.4 seconds)

After that I adapted the existing ones to use the one from composer.

Thanks!

[mfn]

Feedback applied!

[grogy]

Thank you and thanks @jrfnl and @villfa for comments.