Support different types in the parameter store

The test framework stores size_t and int32_t values in the parameter store
by converting them all to int. This is ok in practice, since we assume int
covers int32_t and we don't have test data larger than 2GB. But it's
confusing and error-prone. So make the parameter store a union, which allows
size_t values not to be potentially truncated and makes the code a little
clearer.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/suites/helpers.function

@@ -2,6 +2,7 @@
 /*----------------------------------------------------------------------------*/
 /* Headers */
 
+#include <test/arguments.h>
 #include <test/helpers.h>
 #include <test/macros.h>
 #include <test/random.h>