break up raises block in test

[jreiberkyle]

This raises block has two function calls. So it will pass even if one of the functions does not raise the error. These functions should be checked in two separate raises blocks.

https://github.com/planetlabs/planet-client-python/blob/ff6e62db1962e8530f7be9135b096f1df8516f92/tests/unit/test_specs.py#L50