Make slot optional in SnapConnect

https://github.com/canonical/matter-snap-testing/blob/abae29ac5e865f0c5208350bdab63cecb3bdcc5a/utils/snap.go#L80

The slot side can be omitted for system slots.

Without this possibility, the caller needs to pass an empty string:
```go
utils.SnapConnect(t, "chip-tool:avahi-observe", "")
```
or explicitly set the slot:
```go
utils.SnapConnect(t, chipToolSnap+":avahi-observe", ":avahi-observe")
```

The relevant entry point is utils/snap.go around line 80; inspect SnapConnect's current argument handling and its callers. Make the slot side optional while preserving existing forms, then verify the examples in the issue no longer require an empty string.

Written by the indexing model from the issue text.