Fix MatrixStore test fixture importer
Fix MatrixStore test fixture importer

Previously we weren't restricting to import events of type
"prescribing". This happened to be OK in the contexts where we were
using it, but it breaks on the `one_month_of_measures` fixture.

ebmdatalab/openprescribing,ebmdatalab/openprescribing,ebmdatalab/openprescribing,annapowellsmith/openpresc,annapowellsmith/openpresc,annapowellsmith/openpresc,ebmdatalab/openprescribing,annapowellsmith/openpresc