Cypress. Fix case "Actions on polygon" for Firefox.

[dvkruchinin]

Motivation and context

In CI nightly, the unsuccessful completion of the case 16 test. It is reproduced in Firefox browser in headless mode.
In the test, the shape is activated by pointing the cursor at it (mouse events). One of the steps of the shape activation test:

Activation is not performed.
In this patch, I just changed the activation method via sidebar item.

How has this been tested?

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

[bsekachev]

I had the same issue in another test when was writing PR #3417 (test case_37_object_make_copy.js). It also was reproducible only in headless (but in Chrome). I changed the following:

And looks like it helped.
Maybe it helps you to understand the root cause (I didn't understand).