test-install: Allow the installer to move the install tree aside.

When the install script is successful, one of the final things it
wants to do is to move the tree that Zulip was installed from into the
deployments directory.  It can't do that, at least not in a naive way
with `mv`, if the tree is actually a mount point.  So, stick the tree
inside some other directory that we create just for the purpose of
being the mount point and containing the install tree.

tools/test-install/install

@@ -44,13 +44,13 @@ if [ -d "$INSTALLER" ]; then
     installer_dir="$(readlink -f $INSTALLER)"
 else
     installer_dir="$(mktemp -d --tmpdir zulip-server-XXXXX)"
-    tar -xf "$INSTALLER" -C "$installer_dir" --strip-components=1
+    tar -xf "$INSTALLER" -C "$installer_dir" --transform='s,^[^/]*,zulip-server,'
 fi
 
 mkdir -p /srv/zulip/test-install/pip-cache
 
 lxc-copy --ephemeral --keepdata -n "$BASE_CONTAINER_NAME" -N "$CONTAINER_NAME" \
-         -m bind="$installer_dir":/tmp/zulip-server:ro,bind=/srv/zulip/test-install/pip-cache:/root/.cache/pip
+         -m overlay="$installer_dir":/tmp/src/,bind=/srv/zulip/test-install/pip-cache:/root/.cache/pip
 
 run() {
     lxc-attach -n "$CONTAINER_NAME" -- "$@"
@@ -71,7 +71,7 @@ if [ -z "ok" ]; then
     exit 1
 fi
 
-run /tmp/zulip-server/scripts/setup/install --snakeoil-cert "${INSTALLER_ARGS[@]}"
+run /tmp/src/zulip-server/scripts/setup/install --snakeoil-cert "${INSTALLER_ARGS[@]}"
 # TODO install ends as a zombie (workaround: `sudo ps aux | grep lxc-attach`, kill that)
 
 # TODO settings.py, initialize-database, create realm