Each build module parsed its own test runner's output, so a fix in one never reached the others: a failed node --test
run was worded by a passing line of its first file. buildstep.Failure names the test, its file, its cause and its own
output; ReadGoTest (moved from mesh-build-go) and ReadTAP fill it; package conformance holds every build module to it
with runner output captured with the build nodes' own toolchains.
