net: flaky TestUnixAndUnixpacketServer

[bradfitz]

Fix this flaky test:

http://build.golang.org/log/c1ce49888891e1022cdd9eacfb765eb0ce497562

--- FAIL: TestUnixAndUnixpacketServer (0.00s)
    server_test.go:195: #3: EOF
FAIL
FAIL    net 1.989s

@mikioh, care to look into it?
@aclements, is it easy for you to compute stats on this flakiness?

[aclements]

@aclements, is it easy for you to compute stats on this flakiness?

Very! But they're not very interesting. It appears this is the only time this has happened.

$ findflakes -grep TestUnixAndUnixpacketServer
First observed 2fdff95 10 Nov 20:22 2015 (12 commits ago)
Last observed 2fdff95 10 Nov 20:22 2015 (12 commits ago)
Isolated failure
No known past failures

[rsc]

Only happened once, so maybe not worth it / not possible to figure out.

[cherrymui]

Saw one in a trybot failure https://storage.googleapis.com/go-build-log/31bf88a3/linux-amd64_d97e4f3d.log
in CL https://go-review.googlesource.com/c/38131/. The CL itself is unrelated.