We compile each test file to LLVM IR assembly, and then pass that IR to a
dedicated program that can decode LLVM coverage maps and print them in a more
human-readable format. We can then check that output against known-good
snapshots.
This test suite has some advantages over the existing `run-coverage` tests:
- We can test coverage instrumentation without needing to run target binaries.
- We can observe subtle improvements/regressions in the underlying coverage
mappings that don't make a visible difference to coverage reports.
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