Fix instructions for running target ART gtests in `chroot` with Atest.

We currently have to manually build `art_chroot` manually before
invoking `atest ArtGtestsTargetChroot`, because of build system
limitations; adjust the ART testing documentation until we find a way
to address this build system dependency issue.

Test: Render `README.atest.md` as HTML and check the output manually
Bug: 187416712
Bug: 147820692
Change-Id: I5c5d13ca49d65b6737fb58bd3f6778a560087739
1 file changed