tests/acceptance: bump avocado requirements to 76.0

If we want to use @skipUnless decorations on the class we need a
newer version of avocado.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200303150622.20133-9-alex.bennee@linaro.org>

tests/requirements.txt

 # 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==72.0
+avocado-framework==76.0