test: change GitHub actions checkout from v2 to v3

GitHub actions is reporting a warning that actions/checkout@v2 includes
the deprecated Node.js 12. So switch to actions/checkout@v3 which uses
Node.js 16.

As far as I can tell this has no impact on our workflow.

For details see:
https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/
https://github.com/actions/checkout

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>