Make AbstractTest extendable

[claudiu-cristea]

Problem

If a 3rd party, providing task runner commands, wants to extend \OpenEuropa\TaskRunner\Tests\AbstractTest, it cannot reuse most of its methods, such as ::getClassLoader(), ::getFixtureContent(), ::getSandboxRoot(), because they are hardcoding paths. But when openeuropa/task-runner is a dependency, under vendor/, the paths are no more correct.

For instance, the https://github.com/openeuropa/task-runner-changelog project had to override those methods, see https://github.com/openeuropa/task-runner-changelog/blob/bd01ccdf17fe7f9fe8465188d9f0cd7a1f219684/tests/ChangelogCommandTest.php, in order to reuse AbstractTest.

Proposal

Make \OpenEuropa\TaskRunner\Tests\AbstractTest methods inheritable.