Add more detail to securejoin tests

This also checks that some of the correct user interactions happen,
checking we get a joiner event and the verified chat messages.

It also extends the test utils with the ability to distinguish the
different context logs by having them named.

src/context.rs

@@ -64,7 +64,10 @@ pub struct InnerContext {
 
     pub(crate) last_full_folder_scan: Mutex<Option<Instant>>,
 
-    /// Id for this context on the current device.
+    /// ID for this `Context` in the current process.
+    ///
+    /// This allows for multiple `Context`s open in a single process where each context can
+    /// be identified by this ID.
     pub(crate) id: u32,
 
     creation_time: SystemTime,