refactor: Simplify error handling in JSON decoding in tests

[alexandear]

For consistency with other tests https://github.com/search?q=repo%3Agoogle%2Fgo-github%20assertNilError(t%2C%20json.NewDecoder(r.Body).Decode(v))&type=code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.32%. Comparing base (59da3f3) to head (a3bbd3c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #3602 +/- ## ======================================= Coverage 91.32% 91.32% ======================================= Files 184 184 Lines 16143 16143 ======================================= Hits 14742 14742 Misses 1227 1227 Partials 174 174 

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[gmlewis]

Thank you, @alexandear!
LGTM.
Merging.