Switch to a new branch before commiting, commit only what you want then switch back. Doing a git checkout -b new-branch-for-this-bit-of-code is quick and painless if you want to stage things while you're working, doesn't change anything since you're creating a new branch from your current point, and gives you much easier logging, diffing and merging capabilities later on. This is one of the recommended workflows for git, in fact.

You can also commit parts then stash the rest, test, pop the stash, commit more, stash, test, etc, etc. You can add things to commits with --amend, so you can tweak them if you notice something is missing after you added things with git add -p and then commited. I do this a lot when I want to split a commit and use git add -p.

If you really want to test things from somewhere else, use git new-workdir to create a new working dir based off your local clone. Whatever you stash or commit to one, the other will see, since they're really pointing to the same local git clone (if you look at the .git/ dir of a workdir, you'll notice it's all symlinks to the original clone). Just make sure every local workdir you create like this is on a different branch, otherwise it will get confused when you change things on one side (because, of course, they're all pointing to the same local clone, but suddenly the local files won't match). I use this to have a "test workdir", which is based on a clean and permanently updated branch, where I cherry-pick/merge from the work branch into it, build and test, without having to stash on my work branch, reconfigure things and otherwise stop the coding workflow.