[tests] Fix double slashes in tmp PHPUnit path

[symfonyaml]

Context

When we run tests from the ./tests/tmp/cache directory, the built PHPUnit path contains 2 slashes because the MakerTestRunner::getPath() method already prefix with a slash.

Example

                                                                    ⬇️
./tests/tmp/cache/maker_app_7e4ffd082daa1224b6cfdeae54f3e3b2_current//bin/phpunit

Fix

Remove the prefixed slash in bin/phpunit

[jrushlow]

Thank you @symfonyaml

Test failures fixed upstream