Run the child process with -Wignore

This test is not equipped to say anything about what warnings are or are not
emitted.

src/allmydata/test/test_system.py

@@ -2428,7 +2428,9 @@ class SystemTest(SystemTestMixin, RunBinTahoeMixin, unittest.TestCase):
 
         def _run_in_subprocess(ignored, verb, *args, **kwargs):
             stdin = kwargs.get("stdin")
-            env = kwargs.get("env")
+            env = kwargs.get("env", os.environ)
+            # Python warnings from the child process don't matter.
+            env["PYTHONWARNINGS"] = "ignore"
             newargs = ["--node-directory", self.getdir("client0"), verb] + list(args)
             return self.run_bintahoe(newargs, stdin=stdin, env=env)