Make tests for instantiate in mappings.py

[francescalb]

Instantiate is currently being tested indirectly.

New tests should be aded that tests only instantiate and instantiate_all.

Make sure that all argument possibilities are tested, e.g. different ways to set meta.
The example below is taken from the musicode demo https://github.com/EMMC-ASBL/musicode-ote-intop-demo,
whic can provide as inspiration for this test.

molecule = instantiate(
#meta=str(MOL).rstrip("/#"),
meta=MOL,
instances=list(get_referred_instances(coll2)),
triplestore=ts,
allow_incomplete=True,)