Add Unity-based unit test infrastructure (Phase 1 — 117 tests) (#177)

Introduced new system for unit tests based on the Unity framework.

Added tests for some of the simple functions in four different files.

README.md

@@ -1,3 +1,34 @@
 Files for tbaMUD.
 
+## Unit Tests
+
+tbaMUD ships with a C unit-test suite located in the `tests/` directory.
+The suite uses the [Unity](https://github.com/ThrowTheSwitch/Unity) test
+framework (vendored under `tests/vendor/unity/`).
+
+### Quick start
+
+```
+./configure
+cd tests && make test
+```
+
+`make test` builds each test binary, runs it, and writes JUnit XML results to
+`tests/test-results/`.  A summary is printed to the terminal:
+
+```
+[PASS] test_utils
+[PASS] test_random
+[PASS] test_interpreter
+[PASS] test_class
+```
+
+### CI
+
+The GitHub Actions workflow (`.github/workflows/build.yml`) runs `make test`
+on every push and pull request against `master` and publishes a formatted
+report via the `dorny/test-reporter` action.
+
+See [doc/testing.md](doc/testing.md) for full details on adding new tests and
+understanding the test infrastructure.