In a lot of places, we construct an error to End a step, then immediately log
it to the governing test as test fatal. Save ourselves a bit of boilerplate by
putting methods on Step for that.
There are a couple cases this doesn't cover, e.g., where we construct the Step
outside a subtest that wants to fail individually, but it helps enough to pay
for its lines.
Updates #13038
Change-Id: I71f9900942962de16609b6b198d3ba13d6958a5f
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>