os.Stat: returned error on nonexistent path did not satisfy IsNotExist

Add direct test for the problem to make the fix commit clearer.

Noticed while implementing MkdirTemp on mac; the upstream tests for MkdirTemp fail without this.

src/os/stat_unix.go

@@ -19,7 +19,7 @@ func (f *File) Sync() error {
 func statNolog(name string) (FileInfo, error) {
 	var fs fileStat
 	err := ignoringEINTR(func() error {
-		return syscall.Stat(name, &fs.sys)
+		return handleSyscallError(syscall.Stat(name, &fs.sys))
 	})
 	if err != nil {
 		return nil, &PathError{Op: "stat", Path: name, Err: err}
@@ -32,7 +32,7 @@ func statNolog(name string) (FileInfo, error) {
 func lstatNolog(name string) (FileInfo, error) {
 	var fs fileStat
 	err := ignoringEINTR(func() error {
-		return syscall.Lstat(name, &fs.sys)
+		return handleSyscallError(syscall.Lstat(name, &fs.sys))
 	})
 	if err != nil {
 		return nil, &PathError{Op: "lstat", Path: name, Err: err}