Skip some video unit tests on MacOS

[vinnamkim]

Summary

• This is because of the recent CI failure on MacOS.

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

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

[codecov-commenter]

Codecov Report

Base: 78.26% // Head: 78.27% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (06a03e5) compared to base (4624d59).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #825      +/-   ##
===========================================
+ Coverage    78.26%   78.27%   +0.01%     
===========================================
  Files          184      184              
  Lines        23614    23614              
  Branches      4892     4892              
===========================================
+ Hits         18482    18485       +3     
+ Misses        4033     4032       -1     
+ Partials      1099     1097       -2     

Flag	Coverage Δ
macos-11_Python-3.8	77.57% <ø> (?)
ubuntu-20.04_Python-3.8	78.25% <ø> (ø)
windows-2019_Python-3.8	78.19% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datumaro/plugins/splitter.py	88.66% <0.00%> (+0.22%)	⬆️
datumaro/plugins/synthetic_data/utils.py	85.00% <0.00%> (+1.66%)	⬆️

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