This change improves clock management for zcashd by ensuring
that all clock methods (obtaining seconds, milliseconds, and
microseconds since the epoch) agree under testing conditions
using `-mocktime`, and also adds a feature that allows tests
to specify an offset to the system clock; this is useful to
allow comprehensive testing of the "timejacking attack mitigation"
consensus rules.
That is, without having to launch with test_runner.py. There are
several places where the BITCOIND environment variable determines the
executable, but the default is "bitcoind"; change the default to
"src/zcashd". This does require running the test from the top-level
directory.
Also change the environment variable from BITCOIND to ZCASHD. This could
conceivable break someone, but it just makes too much sense not to do.