chore(test): add virtual file system

[mogery]

Summary:
This PR adds a virtual file system to execution_spec, much like described in #1137.

Issue Reference(s):
Fixes #1137
/claim #1137

Build & Testing:

• I ran cargo test successfully.
• I have run ./lint.sh --mode=fix to fix all linting issues raised by ./lint.sh --mode=check.

Checklist:

• I have added relevant unit & integration tests.
• I have updated the documentation accordingly.
• I have performed a self-review of my code.
• PR follows the naming convention of <type>(<optional scope>): <title>

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8e7644d) 88.29% compared to head (c472dfa) 88.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1142      +/-   ##
==========================================
- Coverage   88.29%   88.28%   -0.01%     
==========================================
  Files         105      105              
  Lines       11583    11583              
==========================================
- Hits        10227    10226       -1     
- Misses       1356     1357       +1     

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[mogery]

codecov being weird again