Make tests more DRY

[charles-cowart]

There are tests, other code in qiita_db/test/test_meta_util.py, etc. that are of the form:

With TRN:
sql =
results = execute(sql)
return results

or similar. This could be wrapped in a util function that reduces the amount of code repeats.

[antgonza]

In case it helps, currently there are 171 occurrences of TRN.execute() in 45 files.