make gnl/gnloperation.torture

gnl/common.h:117:E:gnloperation:test_complex_operations:0: (after this point) Test timeout expired

The problem is due to a race in videomixer with:
* one incoming stream EOS'd (i.e. it's not pushing any data) : stream1
* one incoming stream starting to push data (because it was just activated/linked) : stream2
* sending a seek at the same time

The summary is that:
* the newsegment from stream2 arrives *after* the videomixer seek handler has removed the flushing state of collectpads and *before* the seek event was sent on that stream
* videomixer pushes the seek to stream1 (the one that was EOS'd)
 * flush start comes in that other pad
 * flush stop comes in that other pad
 * collectpad thinks the two streams have been properly setup and forwards FLUSH_STOP downstream
 * data from that stream1 will arrive and block in collectpads (waiting for data from the other stream)
* videomixer pushes the seek to stream2
 * flush start comes in that other pad
   * collectpad thinks it's a new flush and forwards it
   * stream1 stops itself ... and never restarts