Start using piecemeal includes in test files

tests/SelfTest/IntrospectiveTests/ToString.tests.cpp

@@ -1,6 +1,6 @@
-#include <catch2/catch.hpp>
-
 #include <catch2/catch_enum_values_registry.h>
+#include <catch2/catch_matchers_vector.h>
+#include <catch2/catch_test_macros.hpp>
 
 enum class EnumClass3 { Value1, Value2, Value3, Value4 };