We very often expect success, and we sometimes want to get the stdout,
too. Let's add a convenience function for that. It saves a lot of
lines of code.
It's useful for tests, scripts, and debugging to be able to use
specific config instead of the user's config. That's especially true
for our automated tests because they didn't have a place to read
config from on Windows before this patch (they read their config from
`{FOLDERID_RoamingAppData}`, which I don't think we can override in
tests).
This patch introduces a `JJ_TIMESTAMP` environment variable that lets
us specify the timestamp to use in tests. It also updates the tests to
use it, which means we get to simplify the tests a lot now that that
the hashes are predictable.
