spanner: unflake TestSessionNotFoundOnPrepareSession

Increase timeout to prevent flaky failures. Also removes a deprecated
comment that no longer applies.

Fixes #1736.

Change-Id: I27e664ca079d353c218fd92197ee0e3bf78af371
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/51730
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hengfeng Li <hengfeng@google.com>
diff --git a/spanner/session_test.go b/spanner/session_test.go
index b137784..6516530 100644
--- a/spanner/session_test.go
+++ b/spanner/session_test.go
@@ -1158,9 +1158,7 @@
 	sp := client.idleSessions
 
 	// Wait until the health checker has tried to write-prepare the sessions.
-	// This will cause the session pool to write some errors to the log that
-	// preparing sessions failed.
-	waitUntil := time.After(time.Second)
+	waitUntil := time.After(5 * time.Second)
 	var numWriteSessions int
 	var numReadSessions int
 waitForPrepare: