Add TestHarness infrastructure and initial smoke test

- Implemented `TestHarness` class for headless editor testing.
- Added utility methods for text insertion, editing, and querying.
- Introduced `test_daily_driver_harness` for verifying basic text buffer operations.
- Updated CMake to include the new test files.

tests/test_daily_driver_harness.cc

@@ -0,0 +1,12 @@
+#include "Test.h"
+
+#include "tests/TestHarness.h"
+
+
+TEST (DailyDriverHarness_Smoke_CanCreateBufferAndInsertText)
+{
+	ktet::TestHarness h;
+
+	ASSERT_TRUE(h.InsertText("hello"));
+	ASSERT_EQ(h.Line(0), std::string("hello"));
+}