add result file for time step 15 of the multiphysics test

there seems to be something fishy going on...
test runner: allow an arbitrary number of reference results

if one of the files matches (fuzzyly, obviously), the test counts as
passed if none matches it counts as failed. this lets us add reference
results for hosts that need the same number of time steps, but for
whatever reason produce a different result. also, the number of time
steps required to produce the result do not matter anymore, as long as
the data is the same (in the fuzzy sense).
fix valgrind complaint in the multiphysics test

I'm not sure whether this fix is correct, but since this problem is
also present in dumux I don't care too much about it...