go: runtime: corruption in tests

#!watchflakes
default <- (log ~ `runtime.throw\(` || log ~ `^\s*fatal error:`) &&
	builder != "plan9-386-0intro" &&
	builder != "plan9-amd64-0intro" &&
	builder != "plan9-arm" &&
	builder != "linux-loong64-3a5000"

This issue gathers flakes caused by runtime.throw, which should “never happen” and usually indicates some kind of corruption.