Make Random*Generators be seedable

This is a necessary step to fix issue with combining multiple
random generators in one test case, with different section nesting.

src/catch2/generators/catch_generators_random.cpp

@@ -0,0 +1,13 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+
+#include <catch2/generators/catch_generators_random.hpp>
+
+#include <catch2/internal/catch_context.hpp>
+
+std::uint32_t Catch::Generators::Detail::getSeed() { return sharedRng()(); }