Prevent time catch up in real time mode

[bcoconni]

This PR fixes the bug reported in the issue #1187.

A new class Timer is added that manages elapsed time. It is inheriting SGPropertyChangeListener so that every time the property simulation/reset is set the timer is properly reset.

The CI workflow is also modified to check that executing the following command takes at least 90 seconds (i.e. 2 runs of 25 seconds and 1 run of 40 seconds):

JSBSim scripts/c172_cruise_8K.xml --realtime --nice

The script c172_cruise_8K.xml is also simplified:

• It runs quicker: the end time and the time that triggers the reset have been shortened.
• The termination event has been removed. Instead the last run is executed until the end time is reached.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 24.96%. Comparing base (330a6e3) to head (b61c15f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1213   +/-   ##
=======================================
  Coverage   24.96%   24.96%           
=======================================
  Files         170      170           
  Lines       19281    19281           
=======================================
  Hits         4814     4814           
  Misses      14467    14467           

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[coasho]

thanks @bcoconni for implementing this fix! The new Timer class with SGPropertyChangeListener handles the reset behavior really well. I tested PR #1213 and it works exactly as expected now.