Bump phpunit/phpunit from 8.5.5 to 8.5.6

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 8.5.5 to 8.5.6.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@8.5.5...8.5.6)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

composer.json

@@ -31,7 +31,7 @@
         "ext-json": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.5.5"
+        "phpunit/phpunit": "^8.5.6"
     },
     "scripts": {
         "test": "cd tests && phpunit -d max_execution_time=0 && cd ..",