net: enable TestDialParallel, TestDialerFallbackDelay and TestDialCancel on Plan 9

TestDialParallel, TestDialerFallbackDelay and TestDialCancel
require dialTCP to support cancellation, which has been
implemented for Plan 9 in CL 22144.

Updates #11225.
Updates #11932.

Change-Id: I3b30a645ef79227dfa519cde8d46c67b72f2485c
Reviewed-on: https://go-review.googlesource.com/22203
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dial_test.go

@@ -241,9 +241,6 @@ func TestDialParallel(t *testing.T) {
 	if !supportsIPv4 || !supportsIPv6 {
 		t.Skip("both IPv4 and IPv6 are required")
 	}
-	if runtime.GOOS == "plan9" {
-		t.Skip("skipping on plan9; cannot cancel dialTCP, golang.org/issue/11225")
-	}
 
 	closedPortDelay, expectClosedPortDelay := dialClosedPort()
 	if closedPortDelay > expectClosedPortDelay {
@@ -486,9 +483,6 @@ func TestDialParallelSpuriousConnection(t *testing.T) {
 	if !supportsIPv4 || !supportsIPv6 {
 		t.Skip("both IPv4 and IPv6 are required")
 	}
-	if runtime.GOOS == "plan9" {
-		t.Skip("skipping on plan9; cannot cancel dialTCP, golang.org/issue/11225")
-	}
 
 	var wg sync.WaitGroup
 	wg.Add(2)
@@ -813,9 +807,8 @@ func TestDialCancel(t *testing.T) {
 		testenv.MustHaveExternalNetwork(t)
 	}
 
-	if runtime.GOOS == "plan9" || runtime.GOOS == "nacl" {
-		// plan9 is not implemented and nacl doesn't have
-		// external network access.
+	if runtime.GOOS == "nacl" {
+		// nacl doesn't have external network access.
 		t.Skipf("skipping on %s", runtime.GOOS)
 	}