Add some initial tests for securejoin

This currently tests the setup-contact full flow and the shortcut
flow.  More securejoin tests are needed but this puts some
infrastructure in place to start writing these.

src/key.rs

@@ -355,7 +355,7 @@ pub async fn store_self_keypair(
 }
 
 /// A key fingerprint
-#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+#[derive(Clone, Eq, PartialEq, Hash)]
 pub struct Fingerprint(Vec<u8>);
 
 impl Fingerprint {
@@ -375,6 +375,14 @@ impl Fingerprint {
     }
 }
 
+impl fmt::Debug for Fingerprint {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Fingerprint")
+            .field("hex", &self.hex())
+            .finish()
+    }
+}
+
 /// Make a human-readable fingerprint.
 impl fmt::Display for Fingerprint {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {