[FLINK-8383][mesos] Disable test-jar shading

[zentol]

What is the purpose of the change

This PR disables the shading of the flink-mesos test jar, which resolves the build-failure we currently see on master.

Note that this is just a bandaid; I don't know why this fails in the first place.

Verifying this change

Check travis build.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[StephanEwen]

+1 to merge for now to fix the build.

[zentol]

merging.