Previously the only roundtrip test was for an empty report.
This test has fake data similar to that found in real reports.
'Metrics' does not round-trip exactly, so a DeepEqual workaround is
added for that.
We want the middle ground between a small compression size, a fast
compression time and a fast decompression time.
Tests suggest that the default compression level is better than the
maximum compression level: although the reports are 4% bigger and
decompress slower, they compress 33% faster.
See discussion on https://github.com/weaveworks/scope/issues/1457#issuecomment-293288682
