Pruning: Relax too strong assertion: PRUNED => !ARMED

[rhaschke]

• Fix Regression with prunning #337
  If two Connect stages are sequenced, both sides can become ARMED.
  However, that means that the wave of PRUNED status updates, shouldn't overwrite a present ARMED state.
• Added a corresponding unit test
• Improved debug output / comments

[codecov]

Codecov Report

Merging #340 (096c671) into master (ca38d11) will decrease coverage by 0.04%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
- Coverage   54.59%   54.55%   -0.03%     
==========================================
  Files         102      102              
  Lines        7832     7843      +11     
==========================================
+ Hits         4275     4278       +3     
- Misses       3557     3565       +8     

Impacted Files	Coverage Δ
core/include/moveit/task_constructor/storage.h	89.70% <ø> (ø)
core/src/stage.cpp	82.93% <ø> (ø)
core/src/storage.cpp	81.17% <40.00%> (-3.86%)	⬇️
core/src/stages/fixed_state.cpp	95.46% <0.00%> (-4.54%)	⬇️

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[captain-yoshi]

This fixes the regression on my end !