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.

.gitignore

@@ -7,6 +7,7 @@ config.status
 src/Makefile
 src/conf.h
 src/util/Makefile
+tests/Makefile
 src/.accepted
 src/depend
 src/util/depend
@@ -93,3 +94,10 @@ src/test/testfile
 .Trashes
 ehthumbs.db
 Thumbs.db
+
+# Unit test binaries (built artifacts)
+tests/test_utils
+tests/test_random
+tests/test_interpreter
+tests/test_class
+tests/test-results/