Intermittent stall in unit tests

There is a race condition where we wind up enabling dynafed globally when running all the unit tests. Then in the `TestChain100Setup` (used by most of the wallet tests, for example), try to PoW up a bunch of blocks, in the line
https://github.com/ElementsProject/elements/blob/88067ddbe4223e43a4c86912928b28b834bbf69e/src/test/test_bitcoin.cpp#L168
which will just run forever. In dynafed `nNonce` doesn't go into the hash.

We should either force dynafed off for these tests, or just replace the PoW loop with a signed-block setup.