initialize the global context to avoid runtime_error upon destruction

[saikishor]

This fixes the issue pointed by @christophfroehlich about the failing tests in the rolling that at the end of the tests upon destruction, the context becomes invalid

[ RUN ] RealtimeClock.get_system_time
 terminate called after throwing an instance of 'std::runtime_error'
 what(): context cannot be slept with because it's invalid

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

@@            Coverage Diff            @@
##             master     #128   +/-   ##
=========================================
  Coverage          ?   45.97%           
=========================================
  Files             ?       12           
  Lines             ?      546           
  Branches          ?      285           
=========================================
  Hits              ?      251           
  Misses            ?       31           
  Partials          ?      264           

Flag	Coverage Δ
unittests	45.97% <0.00%> (?)

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[christophfroehlich]

A simple fix, thanks for digging into this!

[bmagyar]

Thank you!