Test validate script and function directly and add test to CI

[rly]

Motivation

tests/validation/test_validate.py was not being called by any test scripts or CI. This PR adds test_validate.py to the test.py (under unit tests) and fixes it to work with the latest version of the PyNWB validator.

Also update the validator script so that if a namespace is given and found in the cached namespace, the validator uses it properly. Previously, it was not possible to validate an NWB file against a namespace that is not the most restrictive among the cached namespaces.

How to test the behavior?

python test.py

Checklist

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