Fix __create_trace() in StreamConverter_test.py

A "feature" in obspy is that the stats.npts metadata object is not
automatically calculated if a Trace is created by passing in a Stats
object (instead of a simple dictionary). The Trace still contained
the data array, which is probably why this issue wasn't discovered
sooner, but something as simple as trace.times() is broken if npts
is not specified. It's amazing this wasn't discovered sooner, given
how many times traces get "created" throughout geomag-algoriothms.
Probably, we should survey the codebase to make sure this isn't a
problem elsewhere.