os/exec: remove flaky TestExtraFilesFDShuffle test
os/exec: remove flaky TestExtraFilesFDShuffle test

Update issue 5780

R=golang-dev, cshapiro, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/12869049

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