database/sql: fix TestConversions when count > 1
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 <14db39248b80c6688f7e9808ba5055f2e56426f3@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/go,golang/go,codestation/go,nawawi/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,akutz/go,akutz/go,josharian/go,codestation/go,josharian/go,josharian/go,golang/go,nawawi/go,Cofyc/go,codestation/go,CAFxX/go,CAFxX/go,christopher-henderson/Go,Cofyc/go,golang/go,codestation/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,josharian/go,golang/go,codestation/go,Cofyc/go,cloudflare/go,nawawi/go,Cofyc/go,akutz/go,CAFxX/go,Cofyc/go,cloudflare/go,golang/go,codestation/go,cloudflare/go,nawawi/go,cloudflare/go,josharian/go,CAFxX/go,codestation/go,golang/go,codestation/go,Cofyc/go,josharian/go,cloudflare/go,cloudflare/go,Cofyc/go,CAFxX/go,akutz/go,Cofyc/go,akutz/go,golang/go