Allow running specific tests without .php extension

This is only to allow people to run tests using

```
codecept run functional MyCept
```

instead of

```
codecept run functional MyCept.php
```
Better?

```
        // Add the path to the file name.
        $filepath = str_replace([$this->path, '\\'], ['', '/'], $file);

        // If the file doesn't exists AND it has is no .php extension,
        // add the extension.
        if (!file_exists($filepath) && substr($filepath, -strlen(self::PHPEXT)) !== self::PHPEXT) {
            $filepath .= self::PHPEXT;
        }
```