library/test_bitset.m:
Instead of comparing just one bitset module (tree_bitset by default)
against set_ordlist, compare all of them (tree_bitset, sparse_bitset,
and fat_sparse_bitset) against set_ordlist.
tests/hard_coded/test_bitsets.{m,exp}:
Rename this test case from test_tree_bitset to test_bitsets, since
(through library/test_bitset.m) we now test ALL bitset implementations,
not just tree_bitset.
tests/hard_coded/Mmakefile:
Refer to the test by its new name.
