net: convert TestTCPServer to use subtests

My fix in CL 202618 inadvertently violated an invariant in the inner
loop of TestTCPServer (namely, that len(trchs) == i). That causes a
panic when one or more of the channels is omitted due to a flake.

Instead of trying to fix up the test, let's just factor out a subtest
and skip the whole thing if the transceiver's Dial flakes out.

Updates #32919

Change-Id: Ib6f274a44194311c8c5a2faf19f586cc9eccfd4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/202561
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/server_test.go

@@ -56,9 +56,9 @@ func TestTCPServer(t *testing.T) {
 	const N = 3
 
 	for i, tt := range tcpServerTests {
+		t.Run(tt.snet+" "+tt.saddr+"<-"+tt.taddr, func(t *testing.T) {
 			if !testableListenArgs(tt.snet, tt.saddr, tt.taddr) {
-			t.Logf("skipping %s test", tt.snet+" "+tt.saddr+"<-"+tt.taddr)
-			continue
+				t.Skip("not testable")
 			}
 
 			ln, err := Listen(tt.snet, tt.saddr)
@@ -109,8 +109,7 @@ func TestTCPServer(t *testing.T) {
 						// "i/o timeout" errors when dialing address ::1. The errors have not
 						// been observed on newer versions of the OS, so we don't plan to work
 						// around them. See https://golang.org/issue/32919.
-					t.Logf("ignoring error on known-flaky macOS 10.12 builder: %v", err)
-					continue
+						t.Skipf("skipping due to error on known-flaky macOS 10.12 builder: %v", err)
 					}
 					t.Fatal(err)
 				}
@@ -129,6 +128,7 @@ func TestTCPServer(t *testing.T) {
 					t.Errorf("#%d: %v", i, err)
 				}
 			}
+		})
 	}
 }