Add envelope tests

[viktorprogger]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.97%. Comparing base (d0df1ef) to head (e1f2709).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #247      +/-   ##
============================================
+ Coverage     95.73%   95.97%   +0.23%     
- Complexity      356      360       +4     
============================================
  Files            47       47              
  Lines           961      968       +7     
============================================
+ Hits            920      929       +9     
+ Misses           41       39       -2     

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