pytest: Don't skip 'redirected_stdin_error' marker

These tests call functions which inspect the fileno of stdin, but pytest
replaces stdin during testing. We can mock the function which tries to
access the fileno and then run these tests as normal.

BUG=chromium:1062657
TEST=`run_pytest`

Change-Id: I366d36d7aa6dcb1f930c72ce91c4cc79575a5df6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2130761
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/cros_generate_sysroot_unittest.py b/scripts/cros_generate_sysroot_unittest.py
index bdcdff4..d0c16f7 100644
--- a/scripts/cros_generate_sysroot_unittest.py
+++ b/scripts/cros_generate_sysroot_unittest.py
@@ -17,6 +17,7 @@
 from chromite.scripts import cros_generate_sysroot as cros_gen
 from chromite.lib import osutils
 from chromite.lib import partial_mock
+from chromite.lib import sudo
 
 
 Dir = cros_test_lib.Directory
@@ -57,6 +58,7 @@
 
   def setUp(self):
     self.cg_mock = self.StartPatcher(CrosGenMock())
+    self.PatchObject(sudo.SudoKeepAlive, '_IdentifyTTY', return_value='unknown')
 
   @cros_test_lib.pytestmark_redirected_stdin_error
   def testTarballGeneration(self):