Test helper to override value for tests

[dgageot]

I added a helper function to testutil to override variables for tests and restore them at the end.
wdyt about it?

The only issue is that when the variable type changes, there's no compilation error. It's detected when the test runs. If think it's ok.

If you like it, I can modify the whole codebase.

[codecov-io]

Codecov Report

Merging #2147 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2147   +/-   ##
=======================================
  Coverage   56.73%   56.73%           
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  Files         183      183           
  Lines        7902     7902           
=======================================
  Hits         4483     4483           
  Misses       3003     3003           
  Partials      416      416

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[nkubala]

I think this wouldn't work if we ever switched to run these tests in parallel, but I guess what we have right now probably wouldn't work anyway.

+1