TestDriver: Fix -Wconversion warning
Since !431 (merged) we convert an expression that uses sizeof() to an int as a compile-time constant. Some GCC versions warn about this with -Wconversion. Add a cast to avoid the warning.
Since !431 (merged) we convert an expression that uses sizeof() to an int as a compile-time constant. Some GCC versions warn about this with -Wconversion. Add a cast to avoid the warning.