database/sql: make TestTxContextWaitNoDiscard test more robust

database/sql: make TestTxContextWaitNoDiscard test more robust

Similar to CL 385934, rely on waiter trigger instead of the WAIT query
prefix and factor out the common test code.

Fixes #53222

Change-Id: I46efc85ca102b350bb4dbe8e514921e016870ffb
Reviewed-on: https://go-review.googlesource.com/c/go/+/416655
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Dmitri Goutnik <dgoutnik@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>