Fix MIMIC3 unit tests

[EricSchrock]

While working on updating #751 to use test-resources/core/mimic3demo, I noticed that the MIMIC3 unit tests were being skipped because the path to mimic3demo was wrong. It looks like this has been broken since #607 so a few other issues have crept into the MIMIC3 unit tests that needed to be patched up.

[Logiquo]

any particular reason we want to rename .csv.gz file to UPPER CASE? e.g. ADMISSIONS.csv.gz

[EricSchrock]

This file needs to exist since MIMIC3Dataset always loads the icustays table. However, I only included the header row, as actual data isn't needed to pass the tests.

subject_id,intime,icustay_id,first_careunit,dbsource,last_careunit,outtime

[EricSchrock]

any particular reason we want to rename .csv.gz file to UPPER CASE? e.g. ADMISSIONS.csv.gz

@Logiquo mimic3.yaml defines the default file names expected. Currently, it is set to expect upper case.