BuildSourceImage: expose "unpack" as subcommand

This may be a pattern we can do for some of the more useful functions.
But for now, folks can build, push, etc. and once they skopeo copy an
image to their host, they can

```bash
./BuildSourceImage unpack <src> <dest>
```

and it will be nicely exposed in the container build too

Also included, exporting the defaults into the bats test for easier
direct running. Perhaps we can move the srpm caching into bats too, so
the Makefile target isn't strictly required.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

test/helpers.bash

@@ -1,3 +1,8 @@
+#!/bin/bash
+
+export CTR_IMAGE="${CTR_IMAGE:-localhost/containers/buildsourceimage}"
+export CTR_ENGINE="${CTR_ENGINE:-podman}"
+
 function run_ctr() {
 	run $CTR_ENGINE run --security-opt label=disable --rm "$@"
 }