net: handle "dns failure" as errNoSuchHost on Plan 9

CL 31468 added TestLookupNonLDH, which was failing on Plan 9,
because LookupHost was expecting to return errNoSuchHost
on DNS resolution failure, while Plan 9 returned the
"dns failure" string.

In the Plan 9 implementation of lookupHost, we now return
errNoSuchHost instead of the "dns failure" string, so
the behavior is more consistant with other operating systems.

Fixes #17568.

Change-Id: If64f580dc0626a4a4f19e5511ba2ca5daff5f789
Reviewed-on: https://go-review.googlesource.com/31873
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/lookup_plan9.go

@@ -122,6 +122,9 @@ func (*Resolver) lookupHost(ctx context.Context, host string) (addrs []string, e
 	// host names in local network (e.g. from /lib/ndb/local)
 	lines, err := queryCS(ctx, "net", host, "1")
 	if err != nil {
+		if stringsHasSuffix(err.Error(), "dns failure") {
+			err = errNoSuchHost
+		}
 		return
 	}
 loop: