net: skip TestFileFdBlocks if the "unix" network is not supported

This may fix the android failures observed starting at CL 496080, such
as the one in
https://build.golang.org/log/7bfc4bd192e21c02a167d2d6a5649f1a2b63a8f1.

Change-Id: I4e8eaf9890da269bd1758f59a29fa2a8131d8ae6
Reviewed-on: https://go-review.googlesource.com/c/go/+/496955
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

src/net/file_unix_test.go

@@ -14,6 +14,10 @@ import (
 // For backward compatibility, opening a net.Conn, turning it into an os.File,
 // and calling the Fd method should return a blocking descriptor.
 func TestFileFdBlocks(t *testing.T) {
+	if !testableNetwork("unix") {
+		t.Skipf("skipping: unix sockets not supported")
+	}
+
 	ls := newLocalServer(t, "unix")
 	defer ls.teardown()