Make unit tests PSR-4 compliant

[szepeviktor]

What does this implement/fix? Explain your changes.

Fix PSR-4 autoloading.

Does this close any currently open issues?

No.

Any relevant logs, error output, etc?

Output of composer dump-autoload --no-interaction --optimize --strict-psr

@joelambert Please consider using static analysis, even on a low --level.

[szepeviktor]

@joelambert Friendly ping 🏓