net/interfaces: fix test hang on Darwin

This test could hang because the subprocess was blocked on writing to
the stdout pipe if we find the address we're looking for early in the
output.

Updates #cleanup

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I68d82c22a5d782098187ae6d8577e43063b72573

net/interfaces/interfaces_darwin_test.go

@@ -5,6 +5,7 @@
 
 import (
 	"errors"
+	"io"
 	"net/netip"
 	"os/exec"
 	"testing"
@@ -69,6 +70,7 @@ func likelyHomeRouterIPDarwinExec() (ret netip.Addr, netif string, ok bool) {
 		return
 	}
 	defer cmd.Wait()
+	defer io.Copy(io.Discard, stdout) // clear the pipe to prevent hangs
 
 	var f []mem.RO
 	lineread.Reader(stdout, func(lineb []byte) error {