Implement a test framework

[ariel-miculas]

Right now, our only tests are unit tests, which are not enough for more complex scenarios, such as creating an environment in which to test fsverity.
Copying Serge's test suggestion from #67 (comment):

which lxc-create || exit 0
lxcname=$(basename $(mktemp -u))
lxc-create -t download -n $lxcname -- -d alpine -r edge -a amd64
dir=$(mkdir -d)
mkdir -p $dir/oci $dir/mnt
export dir
export lxcname
lxc-usernsexec -s -- << "EOF"
unshare -m -- << "XXX"
set -e
puzzlefs create $HOME/.local/share/lxc/$lxcname/rootfs $dir/oci rfs
puzzlefs mount $dir/oci rfs $dir/mnt
test -e $dir/mnt/etc
diff -Nrup $HOME/.local/share/lxc/$lxcname/rootfs $dir/mnt
XXX
EOF

[ariel-miculas]

There is support for integration tests in rust ecosystem, we already have one in exe/tests/extract.rs

[ariel-miculas]

Another test that could be added:

#!/bin/bash
FIFO=$(mktemp -u)
mkfifo "$FIFO"
target/debug/puzzlefs mount -i $FIFO -f /tmp/oci-simple first_try /tmp/puzzle&
cat $FIFO ; echo
ls /tmp/puzzle