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

tests/ast.c

@@ -0,0 +1,22 @@
+#include "../src/ast.h"
+#include "munit.h"
+
+MunitResult test_ast_node_alloc(const MunitParameter params[], void *data) {
+    (void)params;
+    (void)data;
+
+    ast_node_t *node = nullptr;
+    error_t *err = ast_node_alloc(&node);
+
+    munit_assert_ptr_not_null(node);
+    munit_assert_ptr_null(err);
+
+    ast_node_free(node);
+
+    return MUNIT_OK;
+}
+
+MunitTest ast_tests[] = {
+    {"/node_alloc", test_ast_node_alloc, nullptr, nullptr, MUNIT_TEST_OPTION_NONE, nullptr},
+    {nullptr,       nullptr,             nullptr, nullptr, MUNIT_TEST_OPTION_NONE, nullptr}
+};