CI: fixture for global rank variable reset

[awaelchli]

What does this PR do?

For #6802

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[codecov]

Codecov Report

Merging #6839 (62912af) into master (22a266d) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #6839   +/-   ##
======================================
  Coverage      91%     91%           
======================================
  Files         192     192           
  Lines       12191   12191           
======================================
  Hits        11145   11145           
  Misses       1046    1046           

[carmocca]

Still WIP? Looks good to me

[awaelchli]

can be included as standalone PR or be merged into #6802, what you prefer.

[carmocca]

Standalone is okay

[ananthsub]

thanks @awaelchli !