Suppress the logging spams caused by page_runner_unittests.

tools/telemetry/telemetry/unittest/system_stub.py

Index: tools/telemetry/telemetry/unittest/system_stub.py
diff --git a/tools/telemetry/telemetry/unittest/system_stub.py b/tools/telemetry/telemetry/unittest/system_stub.py
index ec61eb9d78021f3873f4e50106e1b55d078a4a45..f8b803c94150b18f4ca00c06e2e096411c5998d8 100644
--- a/tools/telemetry/telemetry/unittest/system_stub.py
+++ b/tools/telemetry/telemetry/unittest/system_stub.py
@@ -170,13 +170,20 @@ class CloudStorageModuleStub(object):
 class LoggingStub(object):
   def __init__(self):
     self.warnings = []
+    self.errors = []
 
   def info(self, msg, *args):
     pass
 
-  def warn(self, msg, *args):
+  def error(self, msg, *args):
+    self.errors.append(msg % args)
+
+  def warning(self, msg, *args):
     self.warnings.append(msg % args)
 
+  def warn(self, msg, *args):
+    self.warning(msg, *args)
+
 
 class OpenFunctionStub(object):
   class FileStub(object):