integration_test: fix race condition between stderr and stdout
integration_test: fix race condition between stderr and stdout

Split output of stderr and stdout instead of using combined buffer.

Change-Id: I4099ebca15099e91539b3ebab6b410fc49f76ce1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168347
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go