Add a missing comma

[IRDonch]

Motivation and context

Without the comma, the string literals are joined together, so only one test case is run instead of two.

How has this been tested?

N/A

Checklist

• I submit my changes into the develop branch
• [ ] I have added description of my changes into CHANGELOG file
• [ ] I have updated the documentation accordingly
• [ ] I have added tests to cover my changes
• [ ] I have linked related issues (read github docs)
• [ ] I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[nmanovic]

@IRDonch , could you please remove unused import as well in line 10 (django.http)?