runtime/testdata/testprog: use testenv.SyscallIsNotSupported to check syscall.Unshare

syscall.Unshare is the sort of system call that may be blocked in a
container environment, and experience has shown that different
container implementations choose from a variety of different error
codes for blocked syscalls.

In particular, the patch in
https://git.alpinelinux.org/aports/tree/community/go/tests-unshare-enosys.patch
seems to suggest that the container environment used to test the Go
distribution on Alpine Linux returns ENOSYS instead of EPERM.

The existing testenv.SyscallIsNotSupported helper checks for
the kinds of error codes we have seen from containers in practice, so
let's use that here.

For #62053.
Updates #29366.

Change-Id: Ic6755f7224fcdc0cb8b25dde2d6047ceb5c3ffdf
Reviewed-on: https://go-review.googlesource.com/c/go/+/520057
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>

Author: Bryan C. Mills

src/runtime/testdata/testprog/syscalls_linux.go

@@ -7,6 +7,7 @@ package main
 import (
 	"bytes"
 	"fmt"
+	"internal/testenv"
 	"os"
 	"syscall"
 )
@@ -44,11 +45,8 @@ func getcwd() (string, error) {
 
 func unshareFs() error {
 	err := syscall.Unshare(syscall.CLONE_FS)
-	if err != nil {
-		errno, ok := err.(syscall.Errno)
-		if ok && errno == syscall.EPERM {
-			return errNotPermitted
-		}
+	if testenv.SyscallIsNotSupported(err) {
+		return errNotPermitted
 	}
 	return err
 }