tests/acceptance: rename tests acceptance to tests avocado

In the discussion about renaming the `tests/acceptance` [1], the
conclusion was that the folders inside `tests` are related to the
framework running the tests and not directly related to the type of
the tests.

This changes the folder to `tests/avocado` and adjusts the MAKEFILE, the
CI related files and the documentation.

[1] https://lists.gnu.org/archive/html/qemu-devel/2021-05/msg06553.html

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
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