tox: Silence output

Enable the 'CaptureOutput' fixture provided by oslotest by default.

Change-Id: Ib34d8ab411a67816db2e26b49bec75993b5bed56
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -10,6 +10,8 @@ allowlist_externals =
 passenv =
   OS_TEST_DBAPI_ADMIN_CONNECTION
 setenv =
+  OS_STDOUT_CAPTURE=true
+  OS_STDERR_CAPTURE=true
   BASECOMMAND=stestr run
   {postgresql,all}: PIFPAF_POSTGRESQL=pifpaf -g OS_TEST_DBAPI_ADMIN_CONNECTION run postgresql --
   {mysql,all}: PIFPAF_MYSQL=pifpaf -g OS_TEST_DBAPI_ADMIN_CONNECTION run mysql --