Add unit tests for 5 models

[claire-simpson]

Description

I've added unit tests for Annual Water Yield, Carbon, Blue Carbon, Coastal Vulnerability, and Crop Production (regression). These tests assume that the functions we have currently are producing the correct output.

Fixes #1603 (in part)

Checklist

• Updated HISTORY.rst and link to any relevant issue (if these changes are user-facing)
• Updated the user's guide (if needed)
• Tested the Workbench UI (if relevant)

[emlys]

Wow, thanks @claire-simpson! This must have taken a lot of work and it's going to make these test suites much more comprehensive. My only suggestion would be: if we update test suites for more models and find that the helper functions for creating test data, checking results, etc become redundant, should we abstract those out to a test_utilities module? But I don't think that needs to happen in this PR.