Typo in module unit tests

If you run `./test_reframe.py -v --maxfail=1 unittests/test_modules.py -ra`, you get the following line:

```
SKIPPED [20] unittests/test_modules.py:28: {requst.param} not supported
```

The string should be `request.param` and it will be replaced by the actual module system name.