Follow-up coverage for MysqlImport::runIt() file checker

[paul-m]

This is a follow-up to #4359 where we discovered that MysqlImport::runIt() checks for the existence of a file, and this behavior wasn't covered by a test.

This gives us two things to do:

1. Generate test coverage for:

    // Attempt to resolve resource file name from file path.
    if (($file_path = \Drupal::service('file_system')->realpath($this->resource->getFilePath(TRUE))) === FALSE) {
      return $this->setResultError(sprintf(
        'Unable to resolve file name "%s" for resource with identifier "%s".',
        $this->resource->getFilePath(TRUE),
        $this->resource->getUniqueIdentifier())
      );
    }

2. Determine why MysqlImport performs this check on the file, but ImportJob does not. If needed, turn this file check into a separate method on ImportJob and then both subclasses can use it.