This commit adds a new flag to shuffle all the test cases before running
them so tests which require lots of blocks to be mined are less likely
to be run in the same tranch.
The other benefit is this approach provides a more efficient way to
figure which tests are broken since all the differnet backends are
running different tranches in their builds, we can identify more failed
tests in one push.
To make sure we build the exact version of btcd that is referenced in
the project's go.mod file and to not overwrite any binary the user might
already have installed on the system, we compile btcd into an explicit
file in the itest directory.
This should also speed up invocations of "make itest-only" because the
test harness doesn't always compile btcd on its own.
We also fix a bug with the version parsing where adding a "replace"
directive in the go.mod would result in the awk commands to extract the
wrong version. Because we no longer use the DEPGET goal to build and
install btcd, using a replace directive now actually works for itests.
