[Bug]: tests are not running parallel in make buildsystems

[lw64]

Checklist

• [X] I agree to follow the Code of Conduct that this project adheres to.
• [X] I have searched the issue tracker for a bug that matches the one I want to file, without success.

flatpak-builder version

1.2.2

Flatpak version

1.12.5

How to reproduce

When a module with buildsystem make specifies run-tests true.

Expected Behavior

The test should run in parallel. There should be a -j$(nproc) in the make check command.

Actual Behavior

The test are not running in parallel.

Additional Information

No response