net/http: TestTransportConnectionCloseOnResponse failure on openbsd-386-64 builder

[bcmills]

2020-04-13T04:26:57-7242428/openbsd-386-64

Not clear to me whether this is a bug in net/http, a bug in the test itself, or a bug in the builder's network stack.

CC @bradfitz

(@andybons, I'm not sure who to CC about issues that may be OpenBSD-specific — see #28596.)

[gopherbot]

Change https://golang.org/cl/357370 mentions this issue: dashboard: remove OpenBSD 6.4 builders