spanner: Unflake TestMinOpenedSessions
spanner: Unflake TestMinOpenedSessions

TestMinOpenedSessions did not wait until the maintainer
had had a chance to run, causing the test to sometimes
fail if the check for MinOpened was executed after the
last session was deleted, but before the maintainer had
run.

Fixes #1639.

Change-Id: I8dc0a4114834878302278a214d9f235ea8a14e78
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47490
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Noah Dietz <892d271bd3cbb39857a7b053afeb6c28a7075b07@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go