infrastructure: release resources in test fixture destructors (#9522)

#### Brief overview of PR changes/additions
- Convert the raw owning `LuaInterface*` member in the
`TLuaInterfaceTest` and `TVariableEditorTest` Qt-Test fixtures to
`std::unique_ptr<LuaInterface>` so the fixture's destructor frees it.
- `TLuaInterfaceTest` also stops allocating the interface (and a
`lua_State`) twice: members now init to `nullptr`/empty and are
allocated only in `init()`; `cleanup()` resets the interface before
closing the `lua_State`.

#### Motivation for adding to Mudlet
Keeps the test suite leak-clean and clears static-analysis warnings, per
CLAUDE.md's "smart pointers for non-Qt classes".

#### Other info (issues closed, discussion etc)
- Clears 2 CodeQL `cpp/resource-not-released-in-destructor` warnings
(`test/TVariableEditorTest.cpp`, `test/TLuaInterfaceTest.cpp`).
- Also removes a real runtime leak in `TLuaInterfaceTest`: the old
fixture never deleted the `interface` and double-allocated it
(construction + `init()`), leaking a `LuaInterface` per test plus a
construction-time `lua_State`/`LuaInterface`. Verified gone under
LeakSanitizer (old binary leaked, new binary is leak-clean;
`TVariableEditorTest` already deleted its interface so for it this is
modernization).

**Test case:** Build and run `ctest -R
'TLuaInterfaceTest|TVariableEditorTest'` - both pass (TLuaInterfaceTest
4/4, TVariableEditorTest 96 passed/13 skipped). Optionally run
`./test/TLuaInterfaceTest` with `ASAN_OPTIONS=detect_leaks=1` to confirm
no leaks are reported.

test/TLuaInterfaceTest.cpp

@@ -22,6 +22,8 @@
 #include <VarUnit.h>
 #include <QtTest/QtTest>
 
+#include <memory>
+
 extern "C" {
 #if defined(INCLUDE_VERSIONED_LUA_HEADERS)
 #include <lua5.1/lauxlib.h>
@@ -35,26 +37,30 @@ extern "C" {
 }
 
 
-class TVarTest : public QObject {
-Q_OBJECT
+class TVarTest : public QObject
+{
+    Q_OBJECT
 
 private:
-    lua_State* L = luaL_newstate();
-    LuaInterface* interface = new LuaInterface(L);
+    lua_State* L = nullptr;
+    std::unique_ptr<LuaInterface> interface;
 
 private slots: // NOLINT(readability-redundant-access-specifiers)
 
     void init()
     {
         L = luaL_newstate();
-        interface = new LuaInterface(L);
+        interface = std::make_unique<LuaInterface>(L);
     }
 
-    void cleanup() {
+    void cleanup()
+    {
+        interface.reset();
         lua_close(L);
     }
 
-    void execLua(const QString& string) {
+    void execLua(const QString& string)
+    {
         luaL_loadstring(L, string.toUtf8().constData());
         lua_pcall(L, 0, 0, 0);
     }
@@ -84,7 +90,6 @@ private slots: // NOLINT(readability-redundant-access-specifiers)
         QCOMPARE(testVar->getValue(), "1");
         QCOMPARE(testVar->getValueType(), LUA_TNUMBER);
     }
-
 };
 
 #include "TLuaInterfaceTest.moc"