test: increase test coverage in fluido-ir

[kayagokalp]

Added:

1. graph to_dot test.
2. display tests for ir.rs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.27%. Comparing base (7730358) to head (0519ffc).

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #52       +/-   ##
===========================================
+ Coverage   34.29%   49.27%   +14.97%     
===========================================
  Files          20       20               
  Lines        1522     1656      +134     
===========================================
+ Hits          522      816      +294     
+ Misses       1000      840      -160     

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