Start using piecemeal includes in test files

tests/ExtraTests/X01-PrefixedMacros.cpp

@@ -4,7 +4,9 @@
 #include <catch2/catch_default_main.hpp>
 // This won't provide full coverage, but it might be worth checking
 // the other branch as well
-#include <catch2/catch.hpp>
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/catch_matchers_generic.hpp>
+#include <catch2/catch_matchers_string.h>
 
 #include <type_traits>
 #include <stdexcept>