add linkml validation to unit tests where nmdc-schema database records are made

[aclum]

See documentation on how to do this here

We discovered this week that even when using the python classes it is possible that the unit tests make records which would not be accepted by runtime, see #267 for details. Using linkml validation is a way to check these records w/o adding additional runtime API dependencies to the unit tests.

Alternatives considered: use json:validate endpoint to validate records.