Expore running tests on the main target

[josephlr]

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Right now all the tests run on the host. In an ideal world, we would set .cargo/config's build.target to be our custom target and have everything (xbuild, xclippy, xtest, etc...) build for the main target. This is how blog_os does it.

We would have to setup a custom testing framework however, which could be complex. We would also have to figure out an easy way to run the tests without too much hassle (maybe once #6 is done).