Looks like you can't use a schema dump file to operate an in-memory database while testing

https://laravel.com/docs/9.x/migrations#squashing-migrations

You can try this

DB::unprepared(file_get_contents("path/file.sql"));

Only try it as a last resort. Personally, it is recommended to migrate in the test environment. If you adopt this method, you should also add a check for migration in the test environment

I finally figured it out.

Cause of the problem

The problem lies in the incorrect settings of the test environment. I didn't find the exact cause, but I found how to set up the test environment so that the dump file is found and loaded.

How do I track down errors

This describes the steps I took to find the solution.

In database.php I copied the test database instead of the normal database

• In database.php I have the main database connection:

'mysql' => [
                'driver' => 'mysql',
                'url' => env('DATABASE_URL'),
                'host' => env('DB_HOST', '127.0.0.1'),
                'port' => env('DB_PORT', '3306'),
                'database' => env('DB_DATABASE', 'forge'),
                'username' => env('DB_USERNAME', 'forge'),
                'password' => env('DB_PASSWORD', ''),
                'unix_socket' => env('DB_SOCKET', ''),
                'charset' => 'utf8mb4',
                'collation' => 'utf8mb4_unicode_ci',
                'prefix' => '',
                'prefix_indexes' => true,
                'strict' => false,
                'engine' => null,
                'options' => extension_loaded('pdo_mysql') ? array_filter([
                    PDO::MYSQL_ATTR_SSL_CA => env('MYSQL_ATTR_SSL_CA'),
                ]) : [],
            ],

and test connection

'testing' => [
                    'driver' => 'mysql',
                    'host' => env('DB_TEST_HOST', '127.0.0.1'),
                    'port' => env('DB_TEST_PORT', '3306'),
                    'database' => env('DB_TEST_DATABASE', 'forge'),
                    'username' => env('DB_TEST_USERNAME', 'forge'),
                    'password' => env('DB_TEST_PASSWORD', ''),
                ],

• I copied the testing connection data into a new mysql connection just to see if I got the same results on the command line
• So, the file now looks like this

'mysql' => [
                'url' => env('DATABASE_URL'),
                'driver' => 'mysql',
                    'host' => env('DB_TEST_HOST', '127.0.0.1'),
                    'port' => env('DB_TEST_PORT', '3306'),
                    'database' => env('DB_TEST_DATABASE', 'forge'),
                    'username' => env('DB_TEST_USERNAME', 'forge'),
                    'password' => env('DB_TEST_PASSWORD', ''),
                'unix_socket' => env('DB_SOCKET', ''),
                'charset' => 'utf8mb4',
                'collation' => 'utf8mb4_unicode_ci',
                'prefix' => '',
                'prefix_indexes' => true,
                'strict' => false,
                'engine' => null,
                'options' => extension_loaded('pdo_mysql') ? array_filter([
                    PDO::MYSQL_ATTR_SSL_CA => env('MYSQL_ATTR_SSL_CA'),
                ]) : [],
            ],
    
    /*'testing' => [
                    'driver' => 'mysql',
                    'host' => env('DB_TEST_HOST', '127.0.0.1'),
                    'port' => env('DB_TEST_PORT', '3306'),
                    'database' => env('DB_TEST_DATABASE', 'forge'),
                    'username' => env('DB_TEST_USERNAME', 'forge'),
                    'password' => env('DB_TEST_PASSWORD', ''),
                ],*/

• On the console, I ran php artisan:migrate
• Database dump file found and loaded
• So, in normal case the dump file is found, but in test case not found
• After some research, I changed the settings of the test environment in phpunit.xml, I will explain it now

Filephpunit.xml

phpunit.xml looks like this (the complete file is not shown here):

<server name="QUEUE_CONNECTION" value="sync"/>
        <server name="SESSION_DRIVER" value="array"/>
        <server name="TELESCOPE_ENABLED" value="false"/>
        <env name="DB_CONNECTION" value="testing"/>     
    </php>
</phpunit>

• So, we can see that the test database connection is defined as a unit test
• On the web I found advice to just set up the database tables instead of changing the entire connection for testing because it's easier
• I tried something like this, so phpunit.xml became

<server name="QUEUE_CONNECTION" value="sync"/>
        <server name="SESSION_DRIVER" value="array"/>
        <server name="TELESCOPE_ENABLED" value="false"/>
        <env name="DB_DATABASE" value="cinema_test"/>   
    </php>
</phpunit>

• I removed the test connection from database.php and removed the related obsolete variables from the .env file
• This fixes the problem, now the dump file is also loaded in the test

in conclusion

While I didn't find the actual cause of Laravel failing to load the dump file, I found a workaround by changing the database name just for testing instead of defining a completely new SQL connection for testing purposes. This fixed the issue and now the database dump file is loaded during testing.