testserver: pass logtostderr when starting multiple nodes

This commit adds the `--logtostderr` command line flag when starting a
cluster with multiple nodes, making the behavior consistent with
single-node clusters. In addition, this makes the change introduced in
68fede3 more useful, as the stderr will include the actual cockroach
logs.

testserver/testserver.go

@@ -577,6 +577,7 @@ func NewTestServer(opts ...TestServerOpt) (TestServer, error) {
 			nodes[i].startCmdArgs = []string{
 				serverArgs.cockroachBinary,
 				startCmd,
+				"--logtostderr",
 				secureOpt,
 				storeArg,
 				fmt.Sprintf(