Feature/improve unit tests coverage

[rutefig]

Added more unit tests to the following modules:

• poly
• ast/query
• dsl
• dsl/cb
• compiler/step_selector
• compiler
• super_circuit

Covers some of these issues:
#157 #102 #105

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.37188% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 64.31%. Comparing base (15f441b) to head (140e815).
Report is 1 commits behind head on main.

❗ Current head 140e815 differs from pull request most recent head 8ad9cc0. Consider uploading reports for the commit 8ad9cc0 to get more accurate results

Files	Patch %	Lines
src/frontend/dsl/mod.rs	88.29%	11 Missing ⚠️
src/frontend/dsl/sc.rs	96.19%	4 Missing ⚠️
src/poly/mod.rs	96.15%	4 Missing ⚠️
src/plonkish/ir/sc.rs	99.21%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #189       +/-   ##
===========================================
+ Coverage   52.81%   64.31%   +11.49%     
===========================================
  Files          26       26               
  Lines        7383     8078      +695     
===========================================
+ Hits         3899     5195     +1296     
+ Misses       3484     2883      -601     

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[rutefig]

@leolara and @qwang98 can you review it please?

[leolara]

check those comments

[leolara]

@rutefig it has conflicts

[leolara]

@rutefig it is approved, but you need to fix clippy and rustfmt before merging