test: add unit tests for LineIndex::line_str

[legendiguess]

No description provided.

[codecov]

Codecov Report

Merging #86 into master will decrease coverage by 0.46%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   74.63%   74.16%   -0.47%     
==========================================
  Files         117      117              
  Lines        8893     8984      +91     
==========================================
+ Hits         6637     6663      +26     
- Misses       2256     2321      +65

Impacted Files	Coverage Δ
crates/mun_hir/src/line_index.rs	89.36% <100%> (+22.69%)	⬆️
crates/mun_abi/src/autogen.rs	52.56% <0%> (-18.75%)	⬇️
crates/mun_hir/src/tests.rs	90.47% <0%> (-9.53%)	⬇️
crates/mun_codegen/src/code_gen.rs	77.27% <0%> (-7.73%)	⬇️
crates/tools/src/lib.rs	69.76% <0%> (-5.24%)	⬇️
crates/mun_target/src/spec.rs	60.46% <0%> (-2.95%)	⬇️
crates/mun_codegen/src/ir/ty.rs	82.85% <0%> (-2.44%)	⬇️
crates/mun_runtime/src/assembly.rs	70.45% <0%> (-1.64%)	⬇️
crates/mun_hir/src/source_id.rs	78.48% <0%> (-1.01%)	⬇️
crates/mun_runtime/src/lib.rs	76.19% <0%> (-0.92%)	⬇️
... and 15 more

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[baszalmstra]

thanks!