python: fail fast on the tests

Do not waste CI time running the rest of the tests
if CI is not going to be green anyway.

deltachat-rpc-client/tox.ini

@@ -6,7 +6,7 @@ envlist =
 
 [testenv]
 commands =
-    pytest {posargs}
+    pytest --exitfirst {posargs}
 setenv =
 # Avoid stack overflow when Rust core is built without optimizations.
     RUST_MIN_STACK=8388608