Organize test suites per-database
Add tests for tables and columns queries
Add -i to `dbu conn` for interactive session
Rework query structure for interpolation

The core idea here is to allow queries to be printed as valid sql while
preserving the ability to use prepared queries internally.  This means having a
well-defined interpolation strategy (simple escape for variables like table
names, literal escape for arguments) and a means of creating a prepared
statement from a query.

A few more specific ideas got slipped in here like before/after statements.
These can be used to manually create more complicated prepared statements and
interactions.