test: refactor ImageMagickHandlerTest

[paulbalandan]

Description
This refactors the test to:

• check all available sources of the convert program
• move the extension_loaded check to the native PHPUnit annotation
• fail gracefully and skip the tests altogether if convert is missing instead of erroring midway through the test, i.e. Esh: /usr/local/bin/convert: No such file or directory

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide