Report receiver: keep its log out of the test output

The receiver logs each report it files in Gitea, or fails to, from a
background thread. Those lines landed in the middle of the test results.
Capture stderr in each test instead, in self.stderr for tests that want
to check it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/report-receiver/test_receiver.py

@@ -3,8 +3,10 @@
 They start the receiver on a free port with a fake Gitea, and send it reports
 with the real `report` client, the way a CoreDesktop machine would."""
 
+import contextlib
 import gzip
 import http.server
+import io
 import json
 import os
 import shutil
@@ -66,6 +68,10 @@ class FakeGitea:
 
 class ReceiverTest(unittest.TestCase):
     def setUp(self):
+        # Keep what the receiver logs ("filed as issue #1", ...) out of the
+        # test results. It's in self.stderr for a test that wants to check it.
+        self.stderr = io.StringIO()
+        self.enterContext(contextlib.redirect_stderr(self.stderr))
         self.tmp = Path(tempfile.mkdtemp())
         self.gitea = FakeGitea()
         self.store = receiver.Store(self.tmp / "state", receiver.Gitea(self.gitea.url, "Federated_Computer/coredesktop-reports", "secret"))