test: check that pruneblockchain RPC fails for future block or timestamp #23796

This PR adds missing test coverage for the pruneblockchain RPC for the case that a future block or timestamp is passed: https://github.com/bitcoin/bitcoin/blob/8c0bd871fcf6c5ff5851ccb18a7bc7554a0484b0/src/rpc/blockchain.cpp#L1101 https://github.com/bitcoin/bitcoin/blob/8c0bd871fcf6c5ff5851ccb18a7bc7554a0484b0/src/rpc/blockchain.cpp#L1111 Note that the test method manual_test gets called twice, once each with use_timestamp set to True/False, respectively. Depending on that, the helper function height either converts the passed block height to the timestamp of that block, or just returns it without modification.

The other tests for failures in this RPC are also changed to be more detailled ("Cannot prune blocks because node is not in prune mode", "Negative block height"), as I don't think there is any value in just checking a sub-string. If there is ever an error with the same sub-string is introduced, it's not clear which error is exactly checked with the test, so it makes sense to be as specific as possible.

tACK 140a49ce5e547a1b520a7cd063af8308184e7cbf