acceptance tests: bump Avocado version to 88.1

Besides some internal changes, new features, and bug fixes, on the QEMU side,
this version fixes the following message seen when running the acceptance
tests: "Error running method "pre_tests" of plugin "fetchasset": 'bytes'
object has no attribute 'encode'".

The release notes are available at
https://avocado-framework.readthedocs.io/en/latest/releases/88_0.html.

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210520204747.210764-2-willianr@redhat.com>
Acked-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>

tests/requirements.txt

@@ -1,5 +1,5 @@
 # Add Python module requirements, one per line, to be installed
 # in the tests/venv Python virtual environment. For more info,
 # refer to: https://pip.pypa.io/en/stable/user_guide/#id1
-avocado-framework==85.0
+avocado-framework==88.1
 pycdlib==1.11.0