[essreflectometry] Estia nexus & mcstas tests overwrite each other's reduced data file #597

While working on #591 I ran into this:

Both estia reduction tests save to the same filename via the output_folder fixture:

• packages/essreflectometry/tests/estia/nexus_data_test.py:183 → estia_reduced_iofq.ort (nexus pipeline)
• packages/essreflectometry/tests/estia/mcstas_data_test.py:226 → estia_reduced_iofq.ort (mcstas pipeline)

When tests run with --file-output=tests_outputs (as the nightly reduced-data publish job does), both write into the same directory in a single pytest run so there is a collision.

@jokasimr