When converttopsbt is called with a signed transaction, it either fails with "TX decode failed" if one or more inputs were segwit, or "Inputs must not have scriptSigs and scriptWitnesses" otherwise.
Since no effort is made by the test to ensure the inputs are segwit or not, avoid checking the exact message used.
The error code is still checked to ensure it is of the correct kind of failure.

ninja
test_runner rpc_psbt