circleci: Fix rest-api unit-test

[peterdelevoryas]

circleci: Fix rest-api unit-test

Summary: rest-api unit tests are failing on the Flake8 step in CircleCI (but not in Sandcastle), this refactors the CircleCI setup to allow Flake8 to pass.

Full context on this fix:

• rest-api unit tests use Flake8 to type check Python 3 type annotations.
• Flake8 uses Python 3 multiprocessing.
• multiprocessing uses POSIX semaphores.
• POSIX semaphores use /dev/shm on Linux (man7)
• /dev/shm is a tmpfs virtual filesystem.
• So, Flake8 needs /dev/shm.
• Unit tests run inside the qemux86-image rootfs using chroot.
• The qemux86-image rootfs doesn't have /dev/shm.
• In Sandcastle, we manually create .../qemux86-image-rootfs/dev/shm and mount a 512MB tmpfs there.
• In CircleCI, we cannot create new tmpfs mounts (Forum thread).
• So, flake8 fails in CircleCI with "permission denied" while trying to mount /dev/shm.

The fix in this diff (#thanks @patrickw3 for suggesting this) is to unpack the qemux86-image rootfs within an existing tmpfs, in this case, /mnt/ramdisk. That way, /dev/shm is automatically a tmpfs.

I've also decoupled the build-qemux86 and unit-test jobs in this diff as well, so that we can run ptest-runner as root, but still build the qemux86-image as a non-root user (which bitbake requires). This is so that we can move the mount command from the rest-api recipe to ptest-runner.

Lastly, to move the /dev/shm mounting step from the rest-api recipe to ptest-runner, I needed to refactor the shell command we use when entering the chroot to run a series of commands everytime we enter the chroot. Everytime we exit the chroot, the mount will disappear.

Test Plan: Making sure CircleCI and Sandcastle unit-tests pass.

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@peterdelevoryas has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

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@peterdelevoryas has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@peterdelevoryas has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@peterdelevoryas has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.