common: core_log_set/get_threshold MT unit tests

[janekmi]

This change is 

[janekmi]

Reviewable status: 0 of 6 files reviewed, all discussions resolved (waiting on @grom72 and @osalyk)

a discussion (no related file):
Having two implementations is pointless. Since ATOMIC_OPERATIONS_SUPPORTED is not properly detected in the build system and effectively conflicting with the existing implementation of util_atomic_store/load() which is not out-of-the-box understood by Helgirnd/DRD, I decided to stick to the existing __sync_bool_compare_and_swap() implementation.

The chosen implementation is properly handled by Valgrind tools making testing just easy.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.04%. Comparing base (ad17ad3) to head (3e5569d).
Report is 3 commits behind head on master.

Additional details and impacted files

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##           master    #6035      +/-   ##
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- Coverage   70.05%   70.04%   -0.02%     
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  Files         133      133              
  Lines       19563    19563              
  Branches     3263     3262       -1     
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- Hits        13704    13702       -2     
- Misses       5859     5861       +2     

[osalyk]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @grom72)

[grom72]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @janekmi)

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src/test/Makefile line 113 at r1 (raw file):

	core_log_internal\
	core_log_max\
	core_log_mt\

Do we want to keep all MT tests in one folder or shall we rename this folder to core_log_threshold_mt?

Code quote:

core_log_mt

[grom72]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @janekmi)