spanner: Unflake and speed up TestTakeFromIdleListChecked
spanner: Unflake and speed up TestTakeFromIdleListChecked

TestTakeFromIdleListChecked contained two 'wait for one second'
statements to ensure that a session would be pinged by the next
call to sessionPool.take(). Those waits have been replaced by
a manual reset of the last ping time of the session, instead of
waiting for the time to pass.

The test also contained a 'wait for one maintenance cycle'
statement. This statement is no longer needed after the update
of the session pool maintainer. (The wait was needed to prevent
the maintainer from deleting the session during its first
cycle.)

Change-Id: I255293ce670018ffb50c538627ff03017b3810bb
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47611
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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