cmake: mark the compression tests as slow
cmake: mark the compression tests as slow

Summary:
The compression test code has a large number of parameterized tests that run
with a large number of parameter combinations.  These tests take many minutes
to run.

Reviewed By: meyering

Differential Revision: D6745364

fbshipit-source-id: dac5d5eb4a786da7be453742fac5f92a9740e468

rklabs/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly,facebook/folly,rklabs/folly,facebook/folly,rklabs/folly