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.

CMakeLists.txt

@@ -298,6 +298,8 @@ if (BUILD_TESTS)
     add_executable(kte_tests
             tests/TestRunner.cc
             tests/Test.h
+            tests/TestHarness.h
+            tests/test_daily_driver_harness.cc
             tests/test_buffer_io.cc
             tests/test_piece_table.cc
             tests/test_search.cc