Allow easy retrieval of RNG seed by the users

This makes it so that they don't need parallel RNG seed passing
infrastructure for randomized data generation (e.g. inputs for
benchmarks).

src/CMakeLists.txt

@@ -137,6 +137,7 @@ set(INTERNAL_HEADERS
     ${SOURCES_DIR}/internal/catch_string_manip.hpp
     ${SOURCES_DIR}/internal/catch_stringref.hpp
     ${SOURCES_DIR}/catch_tag_alias.hpp
+    ${SOURCES_DIR}/catch_get_random_seed.hpp
     ${SOURCES_DIR}/catch_tag_alias_autoregistrar.hpp
     ${SOURCES_DIR}/internal/catch_tag_alias_registry.hpp
     ${SOURCES_DIR}/catch_test_case_info.hpp
@@ -230,6 +231,7 @@ set(IMPL_SOURCES
     ${SOURCES_DIR}/matchers/catch_matchers_predicate.cpp
     ${SOURCES_DIR}/matchers/internal/catch_matchers_impl.cpp
     ${SOURCES_DIR}/catch_tag_alias_autoregistrar.cpp
+    ${SOURCES_DIR}/catch_get_random_seed.cpp
     ${SOURCES_DIR}/internal/catch_decomposer.cpp
     ${SOURCES_DIR}/internal/catch_errno_guard.cpp
     ${SOURCES_DIR}/internal/catch_lazy_expr.cpp