[test] stop trying to hit me and hit me -- #8398

TestSyncEncrypterPool.test_encrypt_doc_and_get_it_back was trying to do an operation and asserting the number of attempts. This test is about putting a doc on encrypter pool and getting it encrypted. If we dont wait for the encryption operation to succeed, then complex trial-and-error happens, but if we just ask twisted to wait for one operation before going to the other, this is not needed.

-- Resolves: #8398 (closed)