Add batch size script argument for standalone tests

[awaelchli]

What does this PR do?

Needed for #11098

The batch size for the standalone tests so far was hardcoded. This PR exposes it as a script argument while keeping the hardcoded value as the default.

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Did you have fun?

I made sure I had fun coding 🙃

cc @carmocca @akihironitta @Borda

[codecov]

Codecov Report

Merging #13841 (e6c5010) into master (c391170) will decrease coverage by 10%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #13841     +/-   ##
=========================================
- Coverage      86%      76%    -10%     
=========================================
  Files         332      332             
  Lines       26049    26214    +165     
=========================================
- Hits        22328    19860   -2468     
- Misses       3721     6354   +2633