Move test `bitcoind`/`electrsd` out of `OnceCell`

`OnceCell` doesn't call `drop`, which makes the spawned
`bitcoind`/`electrsd` instances linger around after our tests have
finished. To fix this, we move them out of `OnceCell` and let every test
that needs them spawn their own instances. This additional let us drop
the `OnceCell` dev dependency.