BenchioMediumTestMultiFile test fails

Test fails at run time. Generated error:
```
Reason: attribute error: epcc-reframe/tests/synth/benchio/benchio_fpp.py:40: 'BenchioMediumTestMultiFile' object has no attribute 'write_dir_prefix'
        self.env_vars["WRITE_DIR"] = os.path.join(self.write_dir_prefix, os.environ["USER"], stagedir_name)
```