test: Assert log warnings and errors (#8457)

Adds `assert_warn`, `assert_error` and `assert_warns_or_errors`
methods to `TestContext`, that let us assert that
a certain warning or error is logged during the test.
Also asserts test logs should not contain any other
errors or warnings.

Adjusts tests accordingly.

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

src/sync.rs

@@ -695,6 +695,9 @@ mod tests {
         bob.recv_msg_trash(&sent_msg).await;
         assert!(!token::exists(&bob, token::Namespace::Auth, "testtoken").await?);
 
+        bob.assert_warn("missing key").await;
+        bob.assert_warn("unencrypted message").await;
+
         Ok(())
     }