update order in SynapsesSTDP feature test

[tnowotny]

I have not yet found a workaround to specifying an explicit update order in the synapsesSTDP feature test (test.features.synapses) in order to have brian2genn pass this test. I thought setting the default schedule in prefs before running the test would work but apparently other devices over-ride withe their own defaults. So for the time being, using the standard test (without explicitly changed update schedule) this test will fail with a ca 8% error in brian2genn.