net: skip socket hungry test on iOS

The iOS builder recently gained access to the GO_BUILDER_NAME
environment variable, which in turn enabled some net tests that
were previously guarded by testenv.Builder() == "". Some such tests
have been disabled because they don't work; others have increased
the pressure on open file descriptors, pushing the low iOS limit of
250.

Since many net tests run with t.Parallel(), the "too many open files"
error hit many different tests, so instead of playing whack-a-mole,
lower the file descriptor demand by skipping the most file
descriptor hungry test, TestTCPSpuriousConnSetupCompletionWithCancel.

Before:

$ GO_BUILDER_NAME=darwin-arm64 GOARCH=arm64 go test -short -v net
...
Socket statistical information:
...
(inet4, stream, default): opened=5245 connected=193 listened=75 accepted=177 closed=5399 openfailed=0 connectfailed=5161 listenfailed=0 acceptfailed=143 closefailed=0
...

After:

$ GO_BUILDER_NAME=darwin-arm64 GOARCH=arm64 go test -short -v net
...
Socket statistical information:
...
(inet4, stream, default): opened=381 connected=194 listened=75 accepted=169 closed=547 openfailed=0 connectfailed=297 listenfailed=0 acceptfailed=134 closefailed=0
...

Fixes #25365 (Hopefully).

Change-Id: I8343de1b687ffb79001a846b1211df7aadd0535b
Reviewed-on: https://go-review.googlesource.com/113095
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

src/net/tcpsock_unix_test.go

@@ -87,6 +87,11 @@ func TestTCPSpuriousConnSetupCompletionWithCancel(t *testing.T) {
 	if testenv.Builder() == "" {
 		testenv.MustHaveExternalNetwork(t)
 	}
+
+	if runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64") {
+		t.Skip("the iOS limit of 250 open file descriptors is too low")
+	}
+
 	defer dnsWaitGroup.Wait()
 	t.Parallel()
 	const tries = 10000