Add a constructor and open that take only a single path argument, and then synthesize the real path names by concatenating ".ndx" and ".dat". The initial use case is writing generic test code that also needs to construct or open the basic btree tree types - btree_set, btree_multiset, btree_map, btree_multimap, with their functions that take a single path argument.
bt_time: abstract away differences in value_type and random value generation, thus enabling testing of more btree container types.
Fix btree_set_index constructor calling wrong open. Fix obsolete comment in btree_bases.hpp. Supply missing functions in index_bases. Add string_view_volume_test to index_unit_test.cpp. Add class=btree_set_index-string_view option (and its underlying test) to bt_time.