Disable rtt_test_vectors test on non-Linux platforms
This test depends on Duration
having a granularity of 1
nanosecond, which is not the case on OSX, and is probably not the
case on other places too.
We can re-enable this once we have a set of test vectors that use more realistic RTTs, and a set of testing code that tolerates some divergence.
Temporary solution for #574.