Replace virtual with override where possible.

Using clang-tidy

test/QueryTest.cpp

@@ -25,8 +25,8 @@ using ::testing::MatchesRegex;
 
 class QueryTest : public ::testing::Test {
   protected:
-    void SetUp() { CZNC::CreateInstance(); }
-    void TearDown() { CZNC::DestroyInstance(); }
+    void SetUp() override { CZNC::CreateInstance(); }
+    void TearDown() override { CZNC::DestroyInstance(); }
 };
 
 TEST_F(QueryTest, Name) {