Fix terminating the ongoing process in securejoin

When securejoin allocated an ongoing process it was never freed
again.  This fixes this and also adds test coverage for the right
ongoing behaviour.

src/securejoin/bobstate.rs

@@ -119,7 +119,9 @@ impl<'a> BobStateHandle<'a> {
     /// Note that the state is only cleared on Drop since otherwise the invariant that the
     /// state is always consistent is violated.  However the "ongoing" prococess is released
     /// here a little bit earlier as this requires access to the Context, which we do not
-    /// have on Drop (Drop can not run asynchronous code).
+    /// have on Drop (Drop can not run asynchronous code).  Stopping the "ongoing" process
+    /// will release [`securejoin`](super::securejoin) which in turn will finally free the
+    /// ongoing process using [`Context::free_ongoing`].
     ///
     /// [`InnerContext::bob`]: crate::context::InnerContext::bob
     /// [`Bob`]: super::Bob