Test generate warning for stoichiometry Mg1

Currently the `pyxtal` tests raise the following warning: 
```
/home/runner/work/structuretoolkit/structuretoolkit/structuretoolkit/build/random.py:104: UserWarning: Groups [193, 194] could not be generated with stoichiometry Mg1!
  warnings.warn(
```
It would be great to adjust the test to not raise a warning anymore. 