Think about moving DBItest code here

[krlmlr]

Simplifies generation of spec.

[krlmlr]

Also allows exposing a method that tests a backend. We need to be able to painlessly run the DBI tests on a user's machine.

[krlmlr]

For now, document workflow of specifying/extending the DBI interface:

• Add tests to DBItest
• Install
• document() DBI
• Spec is built from the vignette

[krlmlr]

Maybe as part of a rewrite.