Reduce test set on Travis CI

[bennycode]

Travis CI stops the build if a task needs more than 10 minutes to finish:

No output has been received in the last 10 minutes, this potentially indicates a stalled build or something wrong with the build itself.

The build has been terminated

Reference: https://travis-ci.org/wireapp/proteus.js/builds/155928680

Therefor we should reduce the test complexity on Travis.

These are the tests which take the most time:

Session

• should handle mass communication (approx. 22786ms)
• pathological case (approx. 115321ms)
• max counter gap (approx. 547940ms)

I think it's worth it putting these test in a single spec file and exclude this one from tests ran on Travis CI.

[bennycode]

Even on my test machine tests fail from time to time because they exceed their timeout limit: