Adds tests for `sig_groebner_basis`

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5e649f2) 84.91% compared to head (aed7bb9) 94.03%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #27 +/- ## ========================================== + Coverage 84.91% 94.03% +9.11% ========================================== Files 15 15 Lines 1233 1240 +7 ========================================== + Hits 1047 1166 +119 + Misses 186 74 -112 ``` | [Files](https://app.codecov.io/gh/algebraic-solving/AlgebraicSolving.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/siggb/linear\_algebra.jl](https://app.codecov.io/gh/algebraic-solving/AlgebraicSolving.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3NpZ2diL2xpbmVhcl9hbGdlYnJhLmps) | `96.93% <ø> (+8.04%)` | :arrow_up: | | [src/siggb/siggb.jl](https://app.codecov.io/gh/algebraic-solving/AlgebraicSolving.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3NpZ2diL3NpZ2diLmps) | `98.46% <100.00%> (+9.93%)` | :arrow_up: | | [src/siggb/symbolic\_pp.jl](https://app.codecov.io/gh/algebraic-solving/AlgebraicSolving.jl/pull/27?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3NpZ2diL3N5bWJvbGljX3BwLmps) | `97.26% <100.00%> (+7.76%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/algebraic-solving/AlgebraicSolving.jl/pull/27/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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