Replace individual test binaries with unified test runner.

- Removed standalone test executables (`test_undo`, `test_buffer_save`, `test_buffer_open_nonexistent_save`, etc.).
- Introduced `kte_tests` as a unified test runner.
- Migrated existing tests to a new minimal, reusable framework in `tests/Test.h`.
- Updated `CMakeLists.txt` to build a single `kte_tests` executable.
- Simplified dependencies, reducing the need for ncurses/GUI in test builds.

tests/test_buffer_io.cc

@@ -0,0 +1,79 @@
+#include "Test.h"
+#include <fstream>
+#include <cstdio>
+#include <string>
+#include "Buffer.h"
+
+static std::string read_all(const std::string &path) {
+    std::ifstream in(path, std::ios::binary);
+    return std::string((std::istreambuf_iterator<char>(in)), std::istreambuf_iterator<char>());
+}
+
+TEST(Buffer_SaveAs_and_Save_new_file) {
+    const std::string path = "./.kte_ut_buffer_io_1.tmp";
+    std::remove(path.c_str());
+
+    Buffer b;
+    // insert two lines
+    b.insert_text(0, 0, std::string("Hello, world!\n"));
+    b.insert_text(1, 0, std::string("Second line\n"));
+
+    std::string err;
+    ASSERT_TRUE(b.SaveAs(path, err));
+    ASSERT_EQ(err.empty(), true);
+
+    // append another line then Save()
+    b.insert_text(2, 0, std::string("Third\n"));
+    b.SetDirty(true);
+    ASSERT_TRUE(b.Save(err));
+    ASSERT_EQ(err.empty(), true);
+
+    std::string got = read_all(path);
+    ASSERT_EQ(got, std::string("Hello, world!\nSecond line\nThird\n"));
+
+    std::remove(path.c_str());
+}
+
+TEST(Buffer_Save_after_Open_existing) {
+    const std::string path = "./.kte_ut_buffer_io_2.tmp";
+    std::remove(path.c_str());
+    {
+        std::ofstream out(path, std::ios::binary);
+        out << "abc\n123\n";
+    }
+
+    Buffer b;
+    std::string err;
+    ASSERT_TRUE(b.OpenFromFile(path, err));
+    ASSERT_EQ(err.empty(), true);
+
+    b.insert_text(2, 0, std::string("tail\n"));
+    b.SetDirty(true);
+    ASSERT_TRUE(b.Save(err));
+    ASSERT_EQ(err.empty(), true);
+
+    std::string got = read_all(path);
+    ASSERT_EQ(got, std::string("abc\n123\ntail\n"));
+    std::remove(path.c_str());
+}
+
+TEST(Buffer_Open_nonexistent_then_SaveAs) {
+    const std::string path = "./.kte_ut_buffer_io_3.tmp";
+    std::remove(path.c_str());
+
+    Buffer b;
+    std::string err;
+    ASSERT_TRUE(b.OpenFromFile(path, err));
+    ASSERT_EQ(err.empty(), true);
+    ASSERT_EQ(b.IsFileBacked(), false);
+
+    b.insert_text(0, 0, std::string("hello, world"));
+    b.insert_text(0, 12, std::string("\n"));
+    b.SetDirty(true);
+    ASSERT_TRUE(b.SaveAs(path, err));
+    ASSERT_EQ(err.empty(), true);
+
+    std::string got = read_all(path);
+    ASSERT_EQ(got, std::string("hello, world\n"));
+    std::remove(path.c_str());
+}