Tests for parity with appmetrics

[tobespc]

Raised from

https://github.com/IBM-Swift/SwiftMetrics/issues/10

[sjanuary]

Basic test suite should test for the existence of:

CPU data, including testing that values are non-zero and reasonable
Environment data, check for certain known fields e.g. os.arch, os.name
Memory data, check non-zero reasonable values
Latency data, check values are reasonable
HTTP data from SwiftMetricsKitura.

Instantiation tests for all classes including SwiftMetricsDash should also be done as a minimum