S3: use mocked boto3 module when running unit tests

The unit tests failed if the real Python boto3 module was not
installed.  We can use the mocked module from tests/test-S3/ to make
them work.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Fixes: commit 5130c43b
Acked-by: Laszlo Ersek <lersek@redhat.com>