home-assistant: rerun tests failing with RuntimeError

The flakyness in home-assistant stems from premature event loop closing,
due to race conditions throughout the test suite.

> RuntimeError: Event loop is closed

They also happen during upstreams testsuite runs, but they silently
ignore them, so I don't expect a fix for them in the near future.

Alleviate our pains by rerunning tests that fail in this particular way.

pkgs/servers/home-assistant/default.nix

@@ -136,6 +136,7 @@ in with py.pkgs; buildPythonApplication rec {
     # test infrastructure
     asynctest
     pytest-aiohttp
+    pytest-rerunfailures
     pytest-xdist
     pytestCheckHook
     requests-mock
@@ -292,7 +293,10 @@ in with py.pkgs; buildPythonApplication rec {
 
   pytestFlagsArray = [
     # limit amout of runners to reduce race conditions
-    "-n 2"
+    "-n auto"
+    # retry racy tests that end in "RuntimeError: Event loop is closed"
+    "--reruns 3"
+    "--only-rerun RuntimeError"
     # assign tests grouped by file to workers
     "--dist loadfile"
     # tests are located in tests/