Created the Peak_lists.test_bug_20873_peak_lists system test to catch bug #20873.
This was reported by Troels E. Linnet (http://gna.org/users/tlinnet) at https://gna.org/bugs/?20873.
The test has been created by copying the user function calls from the original bug report and
slightly modifying them to suite a 'relax_fit' analysis type.
