Add unused-variable regression cases

Cover C++17 maybe_unused locals and static variables, plus static registrations with side-effecting initializers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jketema

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/code2.cpp

@@ -176,3 +176,8 @@ void test_switch_initializer()
 		// ...
 	}
 }
+
+void test_maybe_unused()
+{
+	[[maybe_unused]] int unused; // GOOD: explicitly marked as unused
+}

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedStaticVariables/test.cpp

@@ -1,4 +1,6 @@
 
+// semmle-extractor-options: -std=c++17
+
 int globalVar; // GOOD (not static)
 static int staticVar1; // GOOD (used)
 static int staticVar2; // GOOD (used)
@@ -10,6 +12,17 @@ static __attribute__((__unused__)) int staticVar7; // GOOD (unused but this is e
 const int constVar8 = 8; // $ Alert // BAD (const defaults to static)
 extern const int constVar9 = 9; // GOOD
 static int staticVar10 = 10; // $ SPURIOUS: Alert // GOOD [FALSE POSITIVE] (referenced in a never instantiated template)
+[[maybe_unused]] static int staticVar11; // GOOD (explicitly marked as unused)
+
+static int registrationCount;
+
+int* registerBenchmark()
+{
+	++registrationCount;
+	return &registrationCount;
+}
+
+static int* benchmarkRegistration = registerBenchmark(); // GOOD (initializer has a side effect)
 
 void f()
 {