unix: do not invoke Mkfifo with a relative path in a read-only directory

Updates golang/go#28387

Change-Id: Ibcdc3f9cb3dc43b86b7e7d3539ed592219e54aba
Reviewed-on: https://go-review.googlesource.com/c/162987
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux_test.go

@@ -52,6 +52,7 @@ func TestPpoll(t *testing.T) {
 		t.Skip("mkfifo syscall is not available on android, skipping test")
 	}
 
+	defer chtmpdir(t)()
 	f, cleanup := mktmpfifo(t)
 	defer cleanup()
 

unix/syscall_unix_test.go

@@ -426,6 +426,7 @@ func TestPoll(t *testing.T) {
 		t.Skip("mkfifo syscall is not available on android and iOS, skipping test")
 	}
 
+	defer chtmpdir(t)()
 	f, cleanup := mktmpfifo(t)
 	defer cleanup()