test(translate): cover the baidu retry-exhausted path

The baidu translator had no tests. Cover the behaviour the retry loop is
responsible for: a transient error that recovers on the second attempt,
a non-retryable error that must fail immediately, and every attempt
failing so the exhausted-retry path reports the api error. The last one
fails with KeyError: 'trans_result' before the fix.

Follows the layout of tests/test_youdao_translator.py.