JP-3634: Fix for custom WCS unit test

[melanieclarke]

Resolves JP-3634

Closes #8506

This PR just updates a unit test for using a custom WCS in resampling with a more useful override value.

No change notes, documentation updates, or regression test runs necessary.

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• [ ] ~added entry in CHANGES.rst within the relevant release section~
• [x] updated or added relevant tests
• [ ] ~updated relevant documentation~
• [x] added relevant milestone
• [x] added relevant label(s)
• [ ] ~ran regression tests, post a link to the Jenkins job below.~ How to run regression tests on a PR
• [ ] All comments are resolved
• [ ] Make sure the JIRA ticket is resolved properly

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 60.23%. Comparing base (8381431) to head (38fe171). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8620 +/- ## ======================================= Coverage 60.23% 60.23% ======================================= Files 370 370 Lines 38636 38636 ======================================= Hits 23272 23272 Misses 15364 15364 ```

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