Fix delete-by-id bug

When doing a delete-by-id Solr uses the mnemonic `id` to stand for the
`uniqueKey`.  That is, even if `uniqueKey` is not `id` you still pass
in `id` as the field name to delete-by-id.
Fix --test arg for verify script

Don't delete the `yz_rt.beam` file as it holds common functions for
all riak tests.
Make use of yz_rt where possible

Change some of the tests to use common functions in yz_rt.