[Feature Request] Add unit tests

[njzjz]

Summary

Add unit tests for current modules. Some of them do not have packages installed.

Detailed Description

According to Codecov, the following codes doesn't have unit tests.
system:

• System.__str__
• System.dump
• System.extend
• System.predict (add driver plugin system framework; migrate DP predict to DPDriver #277)
• LabeledSystem.__str__
• MultiSystems.__len__ (fixed by fix two bugs that implemented in #168 #172)
• MultiSystems.__repr__
• MultiSystems.__str__ (fixed by fix two bugs that implemented in #168 #172)
• MultiSystems.from_dir
• MultiSystems.get_nframes (fixed by fix two bugs that implemented in #168 #172)
• MultiSystems.predict
• MultiSystems.pick_atom_idx

formats:

• amber/md: from_system
• ase/structure: to_system (fixed by enable unittests of ase and pymatgen #171)
• ase/structure: to_labeled_system
• pymatgen/structure: to_system (fixed by enable unittests of ase and pymatgen #171)
• pymatgen/structure: to_labeled_system (fixed by enable unittests of ase and pymatgen #171)
• mol: to_bond_order_system
• sdf: to_bond_order_system (Bug fix of bond_order_system dump to sdf file #188)
• siesta/output: from_system
• siesta/aimd_output: from_system