The testbed prepare code downloads some snaps to /tmp and subsequently
copies them to /var/lib/snapd/snap where they can be reused by `snap
install` operations. This patch changes that to use $TESTLIB/cache which
may be present locally and may already have the required files, thus
cutting a significant chunk of network traffic.
Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
