spanner: Unflake TestIntegration_SingleUse
spanner: Unflake TestIntegration_SingleUse

The integration test SingleUse exact_staleness assumed that the
execution time of the integration test would be less than 10
seconds. The tese case now calculates the execution time and adds
10 seconds to that to ensure that the read with exact_staleness
reads a snapshot that is older than the test case itself.

Fixes #1592.

Change-Id: I8b3f4a6d1a5d79040cc5459b97cbecb659fbde4e
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/45850
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