The test currently leads to a failure if in general wallet
support is compiled, but the library for the specified type
(BDB/SQLite) is not, i.e. if started with the
`--legacy-wallet` parameter, but bitcoind is compiled
without BDB support.
Fix this by checking if the specified wallet type (BDB for
legacy wallet, SQLite for descriptor wallet) is available.
Also move the helper `is_specified_wallet_compiled()` to the
test framework's class BitcoinTestFramework first, so it can
be reused.