Limit number of threads spawned for tstOMP and tstatomics

[KineticTheory]

Background

• The goal is to improve robustness of these unit tests. On some of our newer test machines we were using more than 100 threads for these tests and this was causing some stability issues.

Purpose of Pull Request

• Fixes re-git issue #1354

Status

• Reference:
  • Pre-Merge Code Review
  • CDash Status
• Checks
  • Travis/Appveyor CI checks pass
  • Code coverage does not decrease
  • Valgrind test passes
  • Toss3 checks pass
  • Trinitite checks pass
  • Code reviewed/approved, sufficient DbC checks, testing, documentation

[codecov]

Codecov Report

Merging #1131 (93a82f0) into develop (751f28b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #1131   +/-   ##
=======================================
  Coverage     89.0%   89.0%           
=======================================
  Files          374     374           
  Lines        19258   19258           
=======================================
  Hits         17154   17154           
  Misses        2104    2104           

[KineticTheory]

@alexrlongne I added some set_omp_num_thread calls to limit the number of spawned threads. What do you think about this change?

[alexrlongne]

This is a good change, thanks!