Improve accuracy of coverage reports 231 (ZcashFoundation#232)

* Add flag to coverage tool to exclude files we don't need to test (ZcashFoundation#231)

* Add documentation to README for running code coverage tests locally (ZcashFoundation#231)

---------

Co-authored-by: Natalie Eskinazi <[email protected]>

Author: natalieesk

Diff for: .github/workflows/coverage.yaml

@@ -38,10 +38,10 @@ jobs:
         run: cargo install cargo-llvm-cov
 
       - name: Run tests
-        run: cargo llvm-cov --lcov --no-report
+        run: cargo llvm-cov --lcov --no-report --ignore-filename-regex 'tests.rs|benches.rs|gendoc'
 
       - name: Generate coverage report
-        run: cargo llvm-cov --lcov --no-run --output-path lcov.info
+        run: cargo llvm-cov report --lcov --ignore-filename-regex 'tests.rs|benches.rs|gendoc' --output-path lcov.info
 
       - name: Upload coverage report to Codecov
         uses: codecov/[email protected]

Diff for: README.md

@@ -35,3 +35,10 @@ dependency.
 1. Run tests `cargo test`
 2. Run formatter `cargo fmt`
 3. Check linter `cargo clippy` and if you want to automatically fix then run `cargo clippy --fix`
+
+## Coverage
+
+Test coverage checks are performed in the pipeline. This is cofigured here: `.github/workflows/coverage.yaml`
+To run these locally:
+1. Install coverage tool by running `cargo install cargo-llvm-cov`
+2. Run `cargo llvm-cov --ignore-filename-regex 'tests.rs|benches.rs|gendoc'` (you may be asked if you want to install `llvm-tools-preview`, if so type `Y`)