js_checker.py: Restore smoke tests for linting violations.

Restoring test to ensure that document.getElementById() calls are flagged as
style violations, now that these checks are performed via ESLint. These tests
are not meant to be exhaustive, instead they just validate that our PRESUBMIT
integration with ESLint works.

BUG=720034

Review-Url: https://codereview.chromium.org/2917473002
Cr-Commit-Position: refs/heads/master@{#476014}
diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index c269deec..e2e43e3 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py
@@ -27,6 +27,7 @@
     self.files = []
     self.is_committing = False
     self.change = MockChange([])
+    self.presubmit_local_path = os.path.dirname(__file__)
 
   def AffectedFiles(self, file_filter=None, include_deletes=False):
     return self.files
@@ -38,7 +39,7 @@
     return self.files
 
   def PresubmitLocalPath(self):
-    return os.path.dirname(__file__)
+    return self.presubmit_local_path
 
   def ReadFile(self, filename, mode='rU'):
     if hasattr(filename, 'AbsoluteLocalPath'):