Include tests directory in composer create-project (#489)

The `tests/` directory was excluded from `composer create-project` via
`.gitattributes` export-ignore. This prevents plugin developers from
getting:

- Example Behat contexts and test structure
- `tests/TestApplication/.env` with DATABASE_URL configuration
- Test fixtures and integration test examples

Since this skeleton is meant for development, the tests directory should
be included to provide a complete starting point.

Author: TheMilek

.gitattributes

@@ -15,4 +15,3 @@
 /phpunit.xml.dist export-ignore
 /psalm.xml export-ignore
 /symfony.lock export-ignore
-/tests export-ignore