Start using piecemeal includes in test files

tests/SelfTest/IntrospectiveTests/StringManip.tests.cpp

@@ -1,7 +1,7 @@
+#include <catch2/catch_test_macros.hpp>
+#include <catch2/catch_matchers_vector.h>
 #include <catch2/catch_string_manip.h>
 
-#include <catch2/catch.hpp>
-
 static const char * const no_whitespace = "There is no extra whitespace here";
 static const char * const leading_whitespace = " \r \t\n There is no extra whitespace here";
 static const char * const trailing_whitespace = "There is no extra whitespace here \t \n \r ";