[Core] Create ElementMap and tests

Test is a dummy for now. `ElementMap.h` only contains const values.

tests/src/App/ElementMap.cpp

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include "gtest/gtest.h"
+
+#include "App/ElementMap.h"
+
+#include <sstream>
+
+class ElementMapTest : public ::testing::Test {
+};
+
+TEST_F(ElementMapTest, defaultConstruction)
+{
+  // Act
+
+  // Assert
+}