Buildkite Test Analytics: fix failure_expanded
The `failure_expanded` attribute of a test result is an _array_ of
objects, not a single object.
I believe this is to support the possibility of multiple failure reasons
in a single test case, although I'm not sure if that's used anywhere.
https://buildkite.com/docs/test-analytics/importing-json#json-test-results-data-reference-test-result-objects
