database/sql: make tests repeatable with -cpu=n,n

New test added in CL 14611045 causes a deadlock when
running the tests with -cpu=n,n because the fakedb
driver always waits when opening a new connection after
running TestConnectionLeak.  Reset its state after.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/14780043

Committer: Brad Fitzpatrick <bradfitz@golang.org>