ComposerIntegrationTest and ComposerValidateTest fail if run from a composer installed site

Maybe you could try a different command line from parent directory:
./vendor/bin/phpunit -c ./core ./modules/contrib/
or something like that, adapted to your directory structure.

I had the same errors (Composer not found) and what worked for me was ./vendor/bin/phpunit -c ./web/core ./web/modules/contrib/. I also added vendor/bin to my PATH but not sure it helped.

If it works that way then, maybe the online documentation should be updated accordingly...

I'm running into the same issue with this:

./docker/bin/phpunit --colors=always \
	    -c /var/www/html/core/phpunit.xml.dist \
	    --testsuite=unit \
	    --group=$(PROFILE_NAME)

There were 2 PHPUnit errors:

1) Drupal\Tests\Composer\Generator\BuilderTest::testBuilder
The data provider specified for Drupal\Tests\Composer\Generator\BuilderTest::testBuilder is invalid
Class "Drupal\Composer\Composer" not found

/var/www/html/core/tests/Drupal/Tests/Composer/Generator/BuilderTest.php:91

2) Drupal\Tests\ComposerIntegrationTest::testComposerTilde
The data provider specified for Drupal\Tests\ComposerIntegrationTest::testComposerTilde is invalid
The "/var/www/html/composer" directory does not exist.

/var/www/html/core/tests/Drupal/Tests/ComposerIntegrationTest.php:55

The first error is looking for class Drupal\Composer\Composer, which would suggest composer is in the vendor/drupal/composer directory, but it's location in my project is actually vendor/composer/composer.

Ok think I found the issue. For some reason, composer is not bringing in the /composer directory from core, still troubleshooting why.

Ok figured out the issue with my tests, a bit confusing.

I thought this was drupal/core: https://git.drupalcode.org/project/drupal/-/tree/11.1.x?ref_type=heads

But the composer.json in the root of this repo is drupal/drupal. There is another composer.json file in /core, and that's drupal/core. So when we are requiring drupal/core-recommended, that requires drupal/core, and so the other files/directories in the root of that repo are not included.

To fix the issue, I added drupal/drupal to require-dev. This puts drupal/drupal in the vendor directory. I then use a composer post install/update script to move vendor/drupal/drupal/composer/* into html/composer (only if it exists with require-dev).

"scripts": {
        "post-install-cmd": [
            "@copy-composer-directory"
        ],
        "post-update-cmd": [
            "@copy-composer-directory"
        ],
        "copy-composer-directory": [
            "[ -d vendor/drupal/drupal/composer ] && mkdir -p html/composer && cp -R vendor/drupal/drupal/composer/* html/composer/ || echo 'composer directory not found; skipping copy'"
        ],
}

Now the Drupal\Tests\Composer\Generator\BuilderTest class, which requires the /composer directory to be in place, can access the proper drupal verison of composer with the Composer::drupalVersionBranch() method, which is obviously drupal specific.