log/syslog: fix flakey test on slow hosts
log/syslog: fix flakey test on slow hosts

Fixes issue 4467.

The syslog tests can fail if the timeout fires before the data arrives at the mock server. Moving the timeout onto the goroutine that is calling ReadFrom() and always processing the data returned before handling the error should improve the reliability of the test.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/6920047

apparentlymart/golang,dsriram/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,tv42/old-go,dsriram/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,apparentlymart/golang