test: Add python test test_qr_securejoin_broadcast, and fix some small bugs I found on the way

deltachat-rpc-client/tox.ini

@@ -6,7 +6,7 @@ envlist =
 
 [testenv]
 commands =
-    pytest -n6 {posargs}
+    pytest {posargs}
 setenv =
 # Avoid stack overflow when Rust core is built without optimizations.
     RUST_MIN_STACK=8388608
@@ -25,6 +25,6 @@ commands =
     ruff check src/ examples/ tests/
 
 [pytest]
-timeout = 300
+timeout = 30
 log_cli = true
 log_level = debug