## Problem
`TestMigrate/Parallel` flakes with:
```
timeout: can't acquire database lock
```
## Root Cause
The test runs two concurrent `migrations.Up(db)` calls on the same
database. golang-migrate wraps every `Lock()` call with a [15-second
timeout](https://github.com/golang-migrate/migrate/blob/v4.19.0/migrate.go#L29)
(`DefaultLockTimeout`). Our `pgTxnDriver.Lock()` uses
`pg_advisory_xact_lock`, which blocks until the lock is available. With
430+ migrations, the first caller can hold the lock well beyond 15s (the
failing test ran for 25.88s), causing the second caller to hit the
timeout.
## Fix
Set `m.LockTimeout = 2 * time.Minute` after creating the
`migrate.Migrate` instance in `setup()`. Since `pg_advisory_xact_lock`
releases automatically when the transaction commits, there's no risk of
a stuck lock — we just need to wait long enough for a concurrent
migration to finish.
Kyle Carberry
8cc6473736