database/sql: fix TestConversions when count > 1

Provide a fresh conversion table for TestConversions as it gets
modified on each test.

Change-Id: I6e2240d0c3455451271a6879e994b82222c3d44c
Reviewed-on: https://go-review.googlesource.com/89595
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>