Update code for finding #[test] functions.

[hermanventer]

Description

Adapt the code for extracting #[test] functions to work with the MIR produced by the currently supported Rust compiler.

Fixes #1200

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• API change with a documentation update
• Additional test coverage
• Code cleanup or just keeping up with the latest Rustc nightly

How Has This Been Tested?

./validate.sh

[codecov]

Codecov Report

Merging #1239 (0df6047) into main (c999902) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##            main   #1239   +/-   ##
=====================================
  Coverage     74%     74%           
=====================================
  Files         23      23           
  Lines      16773   16770    -3     
=====================================
+ Hits       12518   12519    +1     
+ Misses      4255    4251    -4     

Files Changed	Coverage Δ
checker/src/crate_visitor.rs	73% <100%> (+2%)	⬆️

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