unix: fix TestSCMCredentials after CL 272226
unix: fix TestSCMCredentials after CL 272226

After CL 42765 (*net.UnixConn).ReadMsgUnix returns flags with
MSG_CMSG_CLOEXEC set. Adjust TestSCMCredentials accordingly.

Change-Id: Id7da35bbd94c51433f5f27a0e04827795826c02e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/311390
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,golang/sys,golang/sys