chore: miri ignore file opening tests

[ss2165]

Manually run check: https://github.com/CQCL/hugr/actions/runs/9842869762

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.11%. Comparing base (7bb4130) to head (c220875).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1278   +/-   ##
=======================================
  Coverage   87.11%   87.11%           
=======================================
  Files         103      103           
  Lines       19344    19344           
  Branches    17174    17174           
=======================================
  Hits        16851    16851           
  Misses       1715     1715           
  Partials      778      778           

Flag	Coverage Δ
rust	86.57% <ø> (ø)

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