We were reusing Dumper instance in between testDump subtests. This was
not noticed, as it was only "1" and then "empty" case, because "emtpy"
has no transactions. However in the next patch we'll add another
subcase, and if the dumper instance is not reset, it will think that it
starts from transaction number non-zero, which would differ from fresh
dumper output.

-> Fix it.