Fix multi-threaded environment variable mocking

[ashleyfrieze]

Owing to a limitation of Mockito's mockStatic function, which deliberately limits static mocking to the calling thread, the environment variable mocking has not worked in any other worker threads involved in the test.

This was reported in #45

By switching from using Mockito to using ByteBuddy and mocking the ProcessEnvironment closer to the bytecode, we can overcome this constraint.

[codecov-commenter]

Codecov Report

Patch coverage: 28.57% and project coverage change: -3.56% ⚠️

Comparison is base (3b4ea68) 84.76% compared to head (7c81ec9) 81.20%.

❗ Current head 7c81ec9 differs from pull request most recent head 3af3bb4. Consider uploading reports for the commit 3af3bb4 to get more accurate results

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Additional details and impacted files

@@             Coverage Diff              @@
##               main      #46      +/-   ##
============================================
- Coverage     84.76%   81.20%   -3.56%     
+ Complexity      347      326      -21     
============================================
  Files            46       47       +1     
  Lines           755      761       +6     
  Branches         40       40              
============================================
- Hits            640      618      -22     
- Misses           99      129      +30     
+ Partials         16       14       -2     

Files Changed	Coverage Δ
...re/systemstubs/security/NoExitSecurityManager.java	94.94% <ø> (-0.55%)	⬇️
...ere/systemstubs/stream/input/ThrowAtEndStream.java	81.57% <ø> (ø)
...mstubs/internal/ProcessEnvironmentInterceptor.java	0.00% <0.00%> (ø)
...emstubs/environment/EnvironmentVariableMocker.java	93.02% <100.00%> (+40.39%)	⬆️

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