Set XDEBUG_MODE for coverage in PHPUnit with xdebug v3

After switching to xdebug v3 in 325974c, PHPUnit stopped creating
coverage reports and began emitting the following warning:

> XDEBUG_MODE=coverage or xdebug.mode=coverage has to be set

We set this as an environment variable in the `smr-integration-test`
service to re-enable coverage reports.

See sebastianbergmann/php-code-coverage#834.

docker-compose.yml

@@ -121,6 +121,8 @@ services:
     smr-integration-test:
         <<: *smr-common
         command: /smr/vendor/bin/phpunit test
+        environment:
+            - XDEBUG_MODE=coverage
         volumes:
             - ./config/config.specific.sample.php:/smr/config/config.specific.php:ro
             - ./test.env:/smr/.env:ro