LoadTest fail due to "chmod(): Operation not permitted"
The thing is, the chmod wasn't really working.

It was an issue with the mounted drivers in the WSL Ubuntu installation. Based on Stefanos Petrakis' comment, simply trying to unmount and mount the driver with the proper options enabled wouldn't solve the issue by itself. What solved was a bit more drastic:


Factory reset the ubuntu (how-to);
re-mount the driver, with the metadata option included (source):

sudo umount /mnt/c
sudo mount -t drvfs C: /mnt/c -o metadata,uid=1000,gid=1000
Install php, Apache, and everything.


Only then, the phpunit tests did work properly.