X Settled on tracepoints for testing; XXX something is not ok with go:linkname and relative imports?

For the reference how go:linkname works:

	https://lab.nexedi.com/snippets/224

so there is nice happens-before relation when we receive an event and
start something else after it, not a race.

Not yet to go123/xsync as maybe placing exceptions somewhere besides
tests is not so good - I will keep on thinking.