[release-branch.go1.20] net: skip TestFileFdBlocks if the "unix" network is not supported

This may fix the android failures observed starting at CL 496715, such
as the one in
https://build.golang.org/log/a92cc6d5fa36bc31858631bacf2d8eacd93709a6.

For #60217.

Change-Id: I4e8eaf9890da269bd1758f59a29fa2a8131d8ae6
Reviewed-on: https://go-review.googlesource.com/c/go/+/496955
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
(cherry picked from commit 10fbd92)
Reviewed-on: https://go-review.googlesource.com/c/go/+/499297
Reviewed-by: Bryan Mills <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

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()