Before this commit, attempting to spend in a taker-
side Joinmarket coinjoin, a utxo which is of the
tiemelock type (after its timelock expired), had
a potential to crash, because it would be selected
for the generation of a PoDLE but this cannot be
supported (even with taker-side code changes, the
maker could not reconstruct it).
After this commit, any non-standard wallet-type
scriptPubKey is filtered out of being a candidate
for PoDLE generation.
Also a trivial variable name typo is fixed (too-old).
Also the test framework `DummyWallet` now inherits
from `LegacyWallet` because its utxos are of that
type; a future commit should update to `SegwitWallet`.
Also add tests cases for no custom-script PoDLEs.
Adam Gibson