Little test coverage of the hash table implementation in table.c

[gareth-rees]

In table.c we have a hash table implementation. At the moment the only thing it is used for is management of interned strings in eventtxt.c. However, we would like to use hash tables for core features like finalization guardians (job003953) or even white segment lookup (job003796) so we would like to be confident that it is reliable.

Therefore there needs to be test coverage of the functions in table.c.