👌 Reset SN for testing

[ubmarco]

All SN test cases run in the same Python interpreter.
Each test case should see a SN state as it's loaded from disk.
This PR resets SN after test case execution in the test_app fixture.

The problem was a finding of #1441.

[codecov]

Codecov Report

Attention: Patch coverage is 80.18868% with 21 lines in your changes missing coverage. Please review.

Project coverage is 88.42%. Comparing base (4e10030) to head (eeada5b).
Report is 130 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/directives/needgantt.py	62.50%	3 Missing ⚠️
sphinx_needs/directives/needsequence.py	62.50%	3 Missing ⚠️
sphinx_needs/directives/needextract.py	71.42%	2 Missing ⚠️
sphinx_needs/directives/needlist.py	71.42%	2 Missing ⚠️
sphinx_needs/directives/needpie.py	75.00%	2 Missing ⚠️
sphinx_needs/directives/needservice.py	71.42%	2 Missing ⚠️
sphinx_needs/directives/needtable.py	66.66%	2 Missing ⚠️
sphinx_needs/directives/needbar.py	83.33%	1 Missing ⚠️
sphinx_needs/directives/needextend.py	80.00%	1 Missing ⚠️
sphinx_needs/directives/needimport.py	83.33%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1448      +/-   ##
==========================================
+ Coverage   86.87%   88.42%   +1.54%     
==========================================
  Files          56       62       +6     
  Lines        6532     7332     +800     
==========================================
+ Hits         5675     6483     +808     
+ Misses        857      849       -8     

Flag	Coverage Δ
pytests	88.42% <80.18%> (+1.54%)	⬆️

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