The server binary carried exported names that only tests call, including
raw SQL access and a way to weaken the password hash.
Delete testing.go, GetDB and SetBcryptCost from the shipped code, so
passwords always hash at bcrypt.DefaultCost. Move GetDB, GetOrCreateChannel,
JoinChannel and HasChannelInvite into export_test.go. Delete
GetClientHostInfo, whose remaining check GetLatestClientForSession covers.
Tests in other packages now build the database with db.New and run raw SQL
through dbtest.Exec.
Model: opus-5-5
