Make unit tests runnable on PHP8

[lourot]

Note that a lot of tests are broken at this point:

ERRORS!
Tests: 439, Assertions: 546, Errors: 78.
Script ./vendor/bin/phpunit handling the test event returned with error code 2

but at least they can be executed with PHP8.

[lourot]

Note that in these followup commits I was able to fix all unit tests:

• https://github.com/Frameright/image/commit/f35cd1f17299c7647f7db814bd34e10564b26881
• https://github.com/Frameright/image/commit/eda4fc9f4576cbf79f92d21a6d05b19d39ab2480