add to README about how to skip slow tests
add to README about how to skip slow tests

BIP39 tests are really slow (because, basically, BIP39 is deliberately designed
to be slow because it uses 2048 iterations of PBKDF2), so we explain in the
README how to skip these tests by using the cool new environment variable
TEST_SKIP_SLOW.

martindale/fullnode,reddit/fullnode,ryanxcharles/fullnode,ryanxcharles/fullnode,martindale/fullnode