virttest.utils_test: Add fix_atest_cmd() to utils_test

utils_test.fix_atest_cmd() fixes the command
autotest/cli/atest for the external server tests.

e.g.
1. adding -w autotest server argument;
2. adding .../cli prefix/basedir;
and etc..
Signed-off-by: NChen Cao <kcao@redhat.com>