Fix CDN unit test test_detail_usage_metric

[allmightyspiff]

https://github.com/softlayer/softlayer-python/blob/master/tests/managers/cdn_tests.py#L31

CRITICAL root:__init__.py:186 SoftLayer_Network_CdnMarketplace_Metrics::getMappingUsageMetrics property mismatch, args: expected=(12345, 1571168456, 1573760456, 'aggregate'); actual=(12345, 1571168455, 1573760455, 'aggregate')

unit test can be 1s different if the test run is slow. Change the test to just make sure the API is called perhaps.

[allmightyspiff]

https://travis-ci.org/softlayer/softlayer-python/jobs/612034199?utm_medium=notification&utm_source=github_status