Add composer.lock for integration tests

[PVince81]

Background:

• for some reason I had a very old local composer.lock from a previous run, and phpunit had the wrong version (6)
• running composer install would not make it go up to 9 so I kept getting errors when running tests
• git status did not show that composer.lock was modified because the file is not in git, but also is invisible to git status due to gitignore rules on the "build" folder

So this is a workaround to avoid losing time in the future.
The proper solution is more costy: moving build/integration to test/integration (I wonder if we can symlink those though)