Check user defined owners file at root directory

* When a non-default OWNERS file name is defined by "ownersFileName",
  a project's master branch root directory should contain such a file,
  or an error message is logged. This could find a suspected Gerrit
  server bug that returns wrong value for ownersFileName.
* Minor clean up of test code.

Change-Id: If99aa065134e1dd4166bd68006bfd63275bfdc22