Fix random_shuffle narrowing warnings

Catch passes an RNG which accepts int to random_shuffle. Inside
random_shuffle, the STL tries to call that RNG with the difference_type
of the user provided iterators. For std::vector, this is ptrdiff_t,
which on amd64 builds is wider than int. This triggers a narrowing
warning because the 64 bit difference is being truncated to 32 bits.

Note that this RNG implementation still does not produce a correctly
uniformly shuffled result -- it's currently asserting that std::rand
can produce 1000000 which is false -- but I don't know enough about
how much repeatable shuffles are necessary here, so I'm leaving that
alone for now.

include/internal/catch_test_case_registry_impl.hpp

@@ -26,7 +26,7 @@
 namespace Catch {
 
     struct RandomNumberGenerator {
-        typedef int result_type;
+        typedef std::ptrdiff_t result_type;
 
         result_type operator()( result_type n ) const { return std::rand() % n; }