Add working decryption failure test

Our current decryption failure test is incorrect, and is being removed:

It tests if an absent variable can decrypt a payload, and so it fails for the wrong reason: An error is thrown because `alice` does not exist, not because she cannot decrypt the variable.

Highlighted here:
https://github.com/MetaMask/eth-sig-util/pull/74#issuecomment-572330875

Read PR #74 and its linked comment first; the issue provides no file or test path. Locate the decryption failure test being removed, then ensure the replacement exercises failure to decrypt an existing payload rather than failure caused by a missing variable. Done means the test fails for the intended decryption reason and passes with the corrected behavior.

Written by the indexing model from the issue text.