is_admin: work under more limitations

Author: scivision

+stdlib/is_admin.m

@@ -14,8 +14,11 @@
   identity = System.Security.Principal.WindowsIdentity.GetCurrent();
   principal = System.Security.Principal.WindowsPrincipal(identity);
   y = principal.IsInRole(System.Security.Principal.WindowsBuiltInRole.Administrator);
-else
+elseif stdlib.has_java()
   y = com.sun.security.auth.module.UnixSystem().getUid() == 0;
+else
+  [s, r] = system('id -u');
+  y = s == 0 && strip(r) == "0";
 end
 
 end

test/TestFileImpure.m

@@ -61,9 +61,7 @@ function test_get_pid(tc)
 
 
 function test_is_admin(tc)
-  tc.assumeTrue(ispc() || stdlib.has_java() || stdlib.is_mex_fun("stdlib.is_admin"))
-
-  tc.verifyClass(stdlib.is_admin(), "logical")
+tc.verifyClass(stdlib.is_admin(), "logical")
 end