go test defaults to a ten-minute timeout; this suite needs six to
fifteen. A bare go test ./... panics part way through and names
whichever test was executing at the deadline, which is never the one at
fault. It cost this session two full runs and a wrong report that a
branch was failing.
TestMain refuses in half a second with what to run instead. It only
applies to a whole-package run — -run TestOneThing -timeout 2m is a
reasonable thing to type and is left alone.
CONTRIBUTING asked for the bare command and claimed two minutes; both corrected.
Closes #143