google.auth google.auth.getClient: uses the last configured client settings failed

Note: #3669 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky.

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commit: 89a253dc14d931865a4a35fb9093a39a8804ec63
buildURL: [Build Status](https://source.cloud.google.com/results/invocations/55d7bf28-9f1f-46ba-a462-cecfee3aba8c), [Sponge](http://sponge2/55d7bf28-9f1f-46ba-a462-cecfee3aba8c)
status: failed

Start with the failed build linked in the issue and inspect the google.auth.getClient test referenced in the title. Compare it with the previously closed issue #3669 to understand the failure and expected client-selection behavior. Done means reproducing the failure, resolving the use of the last configured client settings, and confirming the relevant test or build passes.

Vom Indexierungsmodell aus dem Issue-Text verfasst.