Document the test outcome file

docs/architecture/Makefile

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docs/architecture/testing/test-framework.md

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+# Mbed TLS test framework
+
+This document is an overview of the Mbed TLS test framework and test tools.
+
+This document is incomplete. You can help by expanding it.
+
+## Running tests
+
+### Outcome file
+
+#### Generating an outcome file
+
+Unit tests and `ssl-opt.sh` record the outcome of each test case in a **test outcome file**. This feature is enabled if the environment variable `MBEDTLS_TEST_OUTCOME_FILE` is set. Set it to the path of the desired file.
+
+If you run `all.sh --outcome-file test-outcome.csv`, this collects the outcome of all the test cases in `test-outcome.csv`.
+
+#### Outcome file format
+
+The outcome file is in a CSV format using `;` (semicolon) as the delimiter and no quoting. This means that fields may not contain newlines or semicolons. There is no title line.
+
+The outcome file has 6 fields:
+
+* **Platform**: a description of the platform, e.g. `Linux-x86_64` or `Linux-x86_64-gcc7-msan`.
+* **Configuration**: a unique description of the configuration (`config.h`).
+* **Test suite**: `test_suite_xxx` or `ssl-opt`.
+* **Test case**: the description of the test case.
+* **Result**: one of `PASS`, `SKIP` or `FAIL`.
+* **Cause**: more information explaining the result.