Add README.meson file for meson test
I've copied test/README and updated it for meson. The changes are:
- Replace autotools test commands with meson equivalent
- Removed obsolete information
- Document all test options and env vars. This is still incomplete. I grepped for getenv and listed all the env vars but have not completed documenting them as I am unsure about some of them.
- Added some information about creating ref images from CI.