[Finder] Test searching in locations with regex special chars in name

If the location to start searching in contains a regex special char
like + or ? and the path restriction is a regular expression with a start
limitation. No results will be found with at least GnuFindAdapter - e.g.:
```
use Symfony\Component\Finder\Finder;
use Symfony\Component\Finder\Adapter;

mkdir('/tmp/reg+ex/dir/subdir', 0777, true);

$finder = Finder::create()
    ->removeAdapters()
    ->addAdapter(new Adapter\GnuFindAdapter());
$finder->in('/tmp/reg+ex')->path('/^dir/');

print count($finder)."\n";
```

Expected result: 2
Actual result is: 0
[Finder] Escape regex special chars in location for regex searches with '^'

Previously to this commit at least GnuFindAdapter find no result in locations
containing regex special chars (e.g. + or ?) and the path restriction is a
regular expression with a start limitation (the regex starts with ^).