unix: fix or skip failing tests on android and iOS

The mkfifo syscall is disallowed on android and iOS. sched_setaffinity
is disallowed on android. Skip all tests which use them.

/usr/bin does not exist on android. Use /system/bin in TestGetwd
instead, like TestChdirAndGetwd in the os package.

Like linux, android does not support Fchmodat with flags != 0. Adjust
TestFchmodat accordingly.

TestDevices might stat some device files which are not accessible, skip
those.

iOS cannot exec subprocesses, thus skip TestPassFD.

Fixes golang/go#25535

Change-Id: Ic764b9152f0a7b703ad4f47fdb1a9a5e94718154
Reviewed-on: https://go-review.googlesource.com/114395
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_test.go

@@ -32,6 +32,10 @@ func TestIoctlGetInt(t *testing.T) {
 }
 
 func TestPpoll(t *testing.T) {
+	if runtime.GOOS == "android" {
+		t.Skip("mkfifo syscall is not available on android, skipping test")
+	}
+
 	f, cleanup := mktmpfifo(t)
 	defer cleanup()
 
@@ -259,6 +263,9 @@ func TestSchedSetaffinity(t *testing.T) {
 	if runtime.NumCPU() < 2 {
 		t.Skip("skipping setaffinity tests on single CPU system")
 	}
+	if runtime.GOOS == "android" {
+		t.Skip("skipping setaffinity tests on android")
+	}
 
 	err = unix.SchedSetaffinity(0, &newMask)
 	if err != nil {