Maintain test order across multiple test suites

Currently the <xharness> task always starts numbering tasks with order ids from 1 up in order they were executed. This numbering is used in the <xharnessresult> task to order the groups and tasks in the result report. If however multiple xharness suites are collated in the same result dir and processed together into just one report, the ordering gets mangled up as there are several tasks with the same order ids.

Instead the xharness task shold store the last used order id in a file in the result directory which the next suite can pick up and use as the start value for further order ids.