ensure consistent sorting in LTRharvest

This PR switches the sorting function used to sort LTR retrotransposon candidates from the system `qsort()` to the one implemented in GenomeTools. Use of the system specific one resulted in inconsistent results on Mac OS X and Linux platforms. Thanks to @stefan-kurtz for the heads up.
Note that running the tests with this change require updating (i.e. pulling changes from) the official `gttestdata` repository.