[Filesystem] Added a missing test case for Filesystem::dumpFile()

This PR adds a test case for the ``Filesystem::dumpFile()`` method (introduced in #7753).

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | [![Build Status](https://travis-ci.org/jakzal/symfony.png?branch=dumpfile-tests)](https://travis-ci.org/jakzal/symfony)
| Fixed tickets | -
| License       | MIT
| Doc PR        | -