Test multithreading

[masterleinad]

This pull request makes the tester run all tests, that currently work multithreaded, with 3 threads.
We should eventually get rid off all the

dealii::MultithreadInfo::set_thread_limit(1); 

[codecov-io]

Codecov Report

Merging #183 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
+ Coverage    87.3%   87.34%   +0.03%     
==========================================
  Files          53       53              
  Lines        2891     2899       +8     
==========================================
+ Hits         2524     2532       +8     
  Misses        367      367

Impacted Files	Coverage Δ
tests/hierarchy_driver.cc	45.39% <100%> (+0.33%)	⬆️
tests/test_hierarchy.cc	86.31% <100%> (+0.34%)	⬆️

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[dalg24]

Need to create an issue to keep track of these