database/pg/pgtest: invoke WrapDB func in Prepare
database/pg/pgtest: invoke WrapDB func in Prepare

Alter `pgtest.WrapDB` to call the provided closure during
`driver.Conn.Prepare` instead of in the `driver.Stmt` functions.
We can do this now that Go 1.8 properly handles panics in
`driver.Conn.Prepare`:

https://go-review.googlesource.com/#/c/23576/

Closes #758

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain