Re-enable prop_timeouts_io test

[coot]

The prop_timeouts_io withTimeoutSerialNative is failing too often, and hence
was disabled. We should find a proper way of fixing this.

      prop_timeouts_io withTimeoutSerialNative:       FAIL (131.41s)
        *** Failed! Falsified (after 53 tests):
        [(0.01s,0.004s),(0.002s,0.003s),(0.002s,0.009s),(0.003s,0.01s),(0.005s,0s),(0.001s,0.003s),(0.01s,0.003s),(0.004s,0.01s),(0.001s,0.003s),(0.002s,0.005s),(0.004s,0.002s),(0.002s,0.001s),(0.002s,0.005s),(0.009s,0.002s),(0.002s,0.005s),(0.004s,0s),(0.01s,0.005s),(0s,0.003s),(0.001s,0.004s),(0.007s,0s),(0.004s,0.008s),(0.004s,0.004s),(0.002s,0.001s),(0.005s,0.005s),(0.009s,0.003s),(0.004s,0.01s),(0.005s,0.005s),(0.005s,0.004s),(0.005s,0.004s),(0s,0.002s),(0.003s,0.005s),(0s,0.008s),(0.004s,0.001s),(0.005s,0.007s),(0.005s,0.002s),(0s,0s),(0.002s,0s),(0s,0s),(0.004s,0.003s),(0.005s,0.005s),(0.008s,0.003s),(0.001s,0s),(0.004s,0.004s),(0.003s,0.005s),(0.009s,0.003s)]
        failure on timeout test #8
        intendedTimeoutDuration: 0.004s
        intendedActionDuration:  0.01s
        intendedOverallDuration: 0.004s
        actualOverallDuration:   0.129451s
        positiveSchedulingTolerance: 0.1s
        violation of timer sanity property:
          actualOverallDuration <= intendedOverallDuration
                                 + positiveSchedulingTolerance
        Use --quickcheck-replay=342479 to reproduce.