Keep the toast's fade-in, and join the test's threads on failure

From the review: opening the toast popover with .show already set skipped the
fade-in; it now opens hidden and then shows. And the superseded-test test now
joins its threads in finally, so a failure can't leave them running into
cleanup.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/test_link.py

@@ -491,6 +491,9 @@ class Connecting(unittest.TestCase):
             finally:
                 for e in release.values():
                     e.set()
+                for t in tests:
+                    if t.ident:  # started
+                        t.join(10)
         result = self.link.snapshot()["tests"][d["id"]]
         self.assertTrue(result["done"])
         self.assertEqual(result["rows"][0]["detail"], "test 2")