cros_build_lib: Add root user check helpers

Add helper functions, to check root privilege for the user.

BUG=b:218521368
TEST=run_tests lib/ scripts/ service/ cli/

Change-Id: Idd0ad8be3764b323b140d3e334399eef4d21e8a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3491293
Tested-by: Ram Chandrasekar <rchandrasekar@google.com>
Auto-Submit: Ram Chandrasekar <rchandrasekar@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Ram Chandrasekar <rchandrasekar@google.com>
diff --git a/cli/deploy_unittest.py b/cli/deploy_unittest.py
index 29357d5..47ff0d3 100644
--- a/cli/deploy_unittest.py
+++ b/cli/deploy_unittest.py
@@ -313,8 +313,7 @@
 
   def setUp(self):
     # Fake being root to avoid running filesystem commands with sudo_run.
-    self.PatchObject(os, 'getuid', return_value=0)
-    self.PatchObject(os, 'geteuid', return_value=0)
+    self.PatchObject(osutils, 'IsRootUser', return_value=True)
     self._sysroot = os.path.join(self.tempdir, 'sysroot')
     osutils.SafeMakedirs(self._sysroot)
     self.device = ChromiumOSDeviceHandlerFake()