tests: add test for add_analyses function

[GraemeWatt]

I merged PR #808 even though the test coverage decreased by -0.006% since it is within the tolerance of -0.02%. The new uncovered line is:

hepdata/hepdata/ext/opensearch/document_enhancers.py

Lines 116 to 117 in 32b9fb7

	doc["analyses"].append({'type': 'NUISANCE', 'analysis': landing_page_url,
	'filename': os.path.basename(reference.file_location)})

The other lines in add_analyses are tested indirectly, for example, doi_minter_test.py imports ins1748602 that includes a HistFactory file. I couldn't work out an easy way of modifying the existing tests to cover the above line. We could wait until a record is finalised that includes a ProSelecta file, then import it with import_records to cover the above line, but it would be better to add a dedicated test for the add_analyses function that covers all supported analysis types (currently: Rivet, MadAnalysis, HistFactory and NUISANCE).