Motivation & Context
In PR #1931, tests were added to ensure that an invalid plain secret key (seckeys) is rejected even when passed alongside a valid one, exercising the early return in the seckeys loop.
However, the equivalent loop for Taproot keypairs (keypairs in secp256k1_silentpayments_sender_create_outputs, src/modules/silentpayments/main_impl.h:245-258) currently only tests passing a single invalid keypair (t of length 1 at line 276).
Changes
This PR adds test coverage ensuring that an invalid Taproot keypair is properly rejected when passed alongside a valid one (t2[2]). The invalid keypair is tested in both positions (first and second index) so that neither the first nor the last keypair is skipped by the check, symmetrically matching the plain secret key test added in #1931.