test(database) :: test that parameters reach the DB and are cached

[81reap]

Before a bound parameter had to survive a round trip in tests/transactions and in computed_projection_binding_order.sql, but nothing bound one variable twice in a single statement, and nothing asserted that a parameterized page runs as a prepared statement rather than through SQLExecDirect.

Now two tests cover both halves of sqlx-oldapi#57, one of wich covers the INSERT ... SELECT $x WHERE $x IS NOT NULL shape from #1474.

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