Fix upper case to lower case variables and functions for flake compliance in tests/unit/preprocessor/_regrid/test_extract_levels.py

[valeriupredoi]

Description

An oversight from my review of #1269 and the annoying result of flake not running until the PR is merged. I have also added the explicit import of iris.fileformats in tests/unit/preprocessor/_regrid/__init__.py so one can actually manually run each of the regrid tests with pytest (we do have an issue open for this sort of bugger).

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[codecov]

Codecov Report

Merging #1347 (76660ef) into main (f47fd97) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1347   +/-   ##
=======================================
  Coverage   88.16%   88.16%           
=======================================
  Files         194      194           
  Lines        9810     9810           
=======================================
  Hits         8649     8649           
  Misses       1161     1161           

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