Adjust Octave unit tests

@houtanb I also looked at the other failing tests. For the quantile.m tests I am not able to replicate the issue. I guess one needs to check on the machine running the tests.

For the dseries tests, the problem seems to be that Octave's io package cannot handle xls files unless Excel is installed. An easy workaround is saving the four http://www.dynare.org/Datasets/dseries/dynseries_test_data-*.xls files in xlsx format and then adjusting the dseries.m tests 9 to 12 to load those