Add a unit test for invalid memos

[scottschurr]

High Level Overview of Change

While reviewing a different pull request I noted that the code coverage of memos is not as good as it could be. So I wrote a unit test to improve that coverage.

Type of Change

• Tests (You added tests for code that already exists, or your new feature included in this PR)

[ximinez]

Nice clean set of tests.

[gregtatcam]

LGTM.
One note - it didn't build for me with boost 1.80.0 (built with 1.75.0). On OSX I got this type of error:
src/ripple/beast/container/detail/aged_ordered_container.h:1435:53: error: no member named 'comp' in 'boost::intrusive::set_impl<boost::intrusive::bhtraits<beast::detail::aged_ordered_container<false, false, std::string, void, std::chrono::steady_clock, std::lessstd::string, std::allocatorstd::string>::element, boost::intrusive::rbtree_node_traits<void *, false>, boost::intrusive::normal_link, boost::intrusive::dft_tag, 3>, void, beast::detail::aged_ordered_container<false, false, std::string, void, std::chrono::steady_clock, std::lessstd::string, std::allocatorstd::string>::KeyValueCompare, unsigned long, true, void>'
: m_config(other.m_config), m_cont(other.m_cont.comp())

[intelliot]

Successfully built on macOS

[intelliot]

CI failure is 0> #3 failed: NodeToShardRPC_test.cpp(47) which is known to be flaky