Acceptance tests: do not try to reuse packages from the system

The premise behind the original behavior is that it would save people
from downloading Avocado (and other dependencies) if already installed
on the system.  To be honest, I think it's extremely rare that the
same versions described as dependencies will be available on most
systems.  But, the biggest motivations here are that:

 1) Hacking on QEMU in the same system used to develop Avocado leads
    to confusion with regards to the exact bits that are being used;

 2) Not reusing Python packages from system wide installations gives
    extra assurance that the same behavior will be seen from tests run
    on different machines;

With regards to downloads, pip already caches the downloaded wheels
and tarballs under ~/.cache/pip, so there should not be more than
one download even if the venv is destroyed and recreated.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210415215141.1865467-3-crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>

tests/Makefile.include

@@ -96,7 +96,7 @@ AVOCADO_TAGS=$(patsubst %-softmmu,-t arch:%, $(filter %-softmmu,$(TARGETS)))
 
 $(TESTS_VENV_DIR): $(TESTS_VENV_REQ)
 	$(call quiet-command, \
-            $(PYTHON) -m venv --system-site-packages $@, \
+            $(PYTHON) -m venv $@, \
             VENV, $@)
 	$(call quiet-command, \
             $(TESTS_VENV_DIR)/bin/python -m pip -q install -r $(TESTS_VENV_REQ), \