net: make TestGoLookupIPWithResolverConfig robust

It crashes when the node under the test is shaken up.

-- FAIL: TestGoLookupIPWithResolverConfig (11.73s)
panic: interface conversion: error is nil, not *net.DNSError [recovered]
	panic: interface conversion: error is nil, not *net.DNSError

goroutine 23 [running]:
panic(0x2e2620, 0xc820181440)
	/go/src/runtime/panic.go:483 +0x3f3
testing.tRunner.func1(0xc820136d80)
	/go/src/testing/testing.go:467 +0x192
panic(0x2e2620, 0xc820181440)
	/go/src/runtime/panic.go:441 +0x4f6
net.TestGoLookupIPWithResolverConfig(0xc820136d80)
	/go/src/net/dnsclient_unix_test.go:358 +0x7ca
testing.tRunner(0xc820136d80, 0x49ddc0)
	/go/src/testing/testing.go:473 +0x98
created by testing.RunTests
	/go/src/testing/testing.go:582 +0x892
exit status 2

Change-Id: I9631f41a3c73f3269c7e30d679c025ae64d71a98
Reviewed-on: https://go-review.googlesource.com/19870Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/dnsclient_unix_test.go

@@ -355,7 +355,13 @@ func TestGoLookupIPWithResolverConfig(t *testing.T) {
 		}
 		addrs, err := goLookupIP(tt.name)
 		if err != nil {
-			if err, ok := err.(*DNSError); !ok || (err.Name != tt.error.(*DNSError).Name || err.Server != tt.error.(*DNSError).Server || err.IsTimeout != tt.error.(*DNSError).IsTimeout) {
+			// This test uses external network connectivity.
+			// We need to take care with errors on both
+			// DNS message exchange layer and DNS
+			// transport layer because goLookupIP may fail
+			// when the IP connectivty on node under test
+			// gets lost during its run.
+			if err, ok := err.(*DNSError); !ok || tt.error != nil && (err.Name != tt.error.(*DNSError).Name || err.Server != tt.error.(*DNSError).Server || err.IsTimeout != tt.error.(*DNSError).IsTimeout) {
 				t.Errorf("got %v; want %v", err, tt.error)
 			}
 			continue