speed up mvfst build by not building fizz test target
speed up mvfst build by not building fizz test target

Summary:
When consuming fizz as an external target, it's wasteful to build test targets
of fizz.
This should fix the travis build timeout of mvfst in oss:
https://travis-ci.com/facebookincubator/mvfst/builds/114688200

Reviewed By: yangchi

Differential Revision: D15712260

fbshipit-source-id: 2c7c1262bb40962c169f5f922e1343e9ab27410e

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst