collect index formatting tests

It looks like PeriodIndex/TDI/DTI tests are using different rules for what constitutes test_indexing/test_setops.  Double-checking before making a PR:

test_get_loc_* --> test_indexing
test_get_indexer_* --> test_indexing
test_insert_* --> test_indexing
test_where_* --> test_indexing
test_join_* --> test_setops
test_append_* --> test_setops
test_difference_* --> test_setops

Many of these at the moment are in e.g. test_ops or test_period/test_datetime.