Revert "net: mark tests with retry as parallel"

This reverts commit CL 103975 (a9b799a2).

Reason for revert: adds data race, breaks race builders, and Brad forgot
to run the Trybots.

Change-Id: Id227dad7069560dbb3ea978a1dcd77ce1979034e
Reviewed-on: https://go-review.googlesource.com/104015Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/lookup_test.go

@@ -63,7 +63,6 @@ var lookupGoogleSRVTests = []struct {
 var backoffDuration = [...]time.Duration{time.Second, 5 * time.Second, 30 * time.Second}
 
 func TestLookupGoogleSRV(t *testing.T) {
-	t.Parallel()
 	if testenv.Builder() == "" {
 		testenv.MustHaveExternalNetwork(t)
 	}
@@ -110,7 +109,6 @@ var lookupGmailMXTests = []struct {
 }
 
 func TestLookupGmailMX(t *testing.T) {
-	t.Parallel()
 	if testenv.Builder() == "" {
 		testenv.MustHaveExternalNetwork(t)
 	}
@@ -191,7 +189,6 @@ var lookupGmailTXTTests = []struct {
 }
 
 func TestLookupGmailTXT(t *testing.T) {
-	t.Parallel()
 	if testenv.Builder() == "" {
 		testenv.MustHaveExternalNetwork(t)
 	}
@@ -301,7 +298,6 @@ var lookupCNAMETests = []struct {
 }
 
 func TestLookupCNAME(t *testing.T) {
-	t.Parallel()
 	if testenv.Builder() == "" {
 		testenv.MustHaveExternalNetwork(t)
 	}