docs: add code coverage section to test README

Document how contributors can measure code coverage locally with
pytest-cov, link to the project's Codecov dashboard, and note the
~80% coverage goal. Mentions the pyproject.toml configuration and
the LONG_TESTS / EXTERNAL_SYSTEM gating that affects local figures
versus what CI measures.

fixes #4952

Signed-off-by: ABD <abdullahfazili3614@gmail.com>

Author: ABD07xx

doc/test/README.md

@@ -6,6 +6,7 @@ You can see all existing tests in [`test/`](https://github.com/ossf/cve-bin-tool
   - [Running all tests](#running-all-tests)
   - [Running a single test](#running-a-single-test)
   - [Running tests on different versions of Python](#running-tests-on-different-versions-of-python)
+  - [Measuring code coverage](#measuring-code-coverage)
   - [Adding new tests: CVE mapping tests](#adding-new-tests-cve-mapping-tests)
   - [Adding new tests: Signature tests against real files](#adding-new-tests-signature-tests-against-real-files)
   - [Adding new tests: Checker filename mappings](#adding-new-tests-checker-filename-mappings)
@@ -87,6 +88,36 @@ deactivate
 
 ```
 
+## Measuring code coverage
+
+CVE Binary Tool uses [`pytest-cov`](https://pytest-cov.readthedocs.io/) to measure code coverage during test runs, and CI uploads the results to [Codecov](https://codecov.io/gh/ossf/cve-bin-tool). The current goal is around 80% coverage as reported by Codecov.
+
+`pytest-cov` is included in the `[dev]` extras, so if you installed the development dependencies (`pip install .[dev]`) it is already available.
+
+To measure coverage locally, run pytest with the `--cov` flag:
+
+```console
+pytest --cov
+```
+
+For a human-readable HTML report, add `--cov-report=html`:
+
+```console
+pytest --cov --cov-report=html
+```
+
+This writes an `htmlcov/` directory in the repository root; open `htmlcov/index.html` in a browser to browse the report.
+
+Coverage settings live in the `[tool.coverage.run]` and `[tool.coverage.report]` sections of `pyproject.toml`. By default, coverage is collected for the `cve_bin_tool` and `test` packages with branch coverage enabled.
+
+Note that, as with the regular test suite, longer tests are gated behind the `LONG_TESTS` and `EXTERNAL_SYSTEM` environment variables described above. Coverage figures from a default `pytest --cov` run will only reflect the short test suite. To approximate what CI measures, run with the same gating variables and report flags CI uses:
+
+```console
+LONG_TESTS=1 EXTERNAL_SYSTEM=1 pytest --cov --cov-report=xml --cov-append -n 8
+```
+
+Be aware that the long test suite can take a substantial amount of time to complete.
+
 ## Adding new tests: CVE mapping tests
 
 - You can see the code for scanner tests in ['test/test_scanner.py'](https://github.com/ossf/cve-bin-tool/blob/main/test/test_scanner.py)