Flaky test: `HTTPClientTests.testVaryingLoopPreference`

[glbrntt]

HTTPClientTests.testVaryingLoopPreference occasionally fails with:

AsyncHTTPClientTests/HTTPClientTests.swift:1452: error: HTTPClientTests.testVaryingLoopPreference : failed - Unexpected error: invalid constant string
AsyncHTTPClientTests/HTTPClientTests.swift:1452: error: HTTPClientTests.testVaryingLoopPreference : failed - Unexpected error: invalid constant string
AsyncHTTPClientTests/HTTPClientTests.swift:1452: error: HTTPClientTests.testVaryingLoopPreference : failed - Unexpected error: invalid constant string
...