Add initial unit tests

- Add µnit source and header files
- Add test target to the build system
- Implement a thorough lexer test suite
- Implement a minimal AST test suite

make/test.mk

@@ -0,0 +1,9 @@
+.PHONY: test
+
+BUILD_DIR=build/test/
+TARGET=oas-tests
+SOURCES = $(filter-out src/main.c, $(shell find src/ tests/ -type f -name '*.c'))
+-include make/base.mk
+
+test: $(BUILD_DIR)$(TARGET)
+	$(BUILD_DIR)$(TARGET)