How to ignore files only locally in git?

I have automated tests running everyday, and the issue is that those tests make necessary changes to certain files depending on which test is running. This becomes a problem when I have to report the changes in the repo, as this will report untracked changed, which means I have to ignore certain files such that local Git doesn't report changes in them specifically.

By making sure to run git update-index --assume-unchanged ./matplotlibrc after making the addition to the exclude file this means changes won't be picked up until then.

git update-index --assume-unchanged index.rst
git update-index --assume-unchanged matplotlibrc


Before:

After:

Changes made but untracked, nice!!!