tests: Avoid the download template when possible

The use of the download template with an hardcoded --arch=amd64 in aa.c
was causing test failures on any platform incapable of running amd64
binaries.

This wasn't noticed in the CI environment as we run the tests within
containers on an amd64 kernel but this caused failures on the Ubuntu CI
environment.

Instead, let's use the busybox template, tweaking the configuration when
needed to match the needs of the testcase.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>