Fixes bug: The message "[InvalidArgumentException] Test file can't be matched" displayed when exactly one test is running under Windows with the full path to the test.

As I see this bugfix does not break tests.
Tests are broken my regression test:

    public function runOneFileOnWindows(\CliGuy $I)
    {
        $I->wantTo('execute one test');
        $I->executeCommand('run tests\\dummy\\FileExistsCept.php');
        $I->seeInShellOutput("OK (");
    }