test(httpclient): use better assertion for timeout test

Kev Klossrequested to merge
kev-better-timeout-assertion into master

This replaces assert.True with assert.Contains when testing the timeout error message in TestClient_TimeoutIsEnforced.

This shows the (admittedly lowercased) error message instead of just a Should be true message. This is particularly helpful when it fails in CI, such as in https://gitlab.com/gitlab-org/labkit/-/jobs/13954002059.

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