test: Explicitly set mock-insights purpose to CLIENT_AUTH

Otherwise connections to it will silently fail. Hurray!

test/verify/files/mock-insights

@@ -210,7 +210,7 @@ def insights_server(port):
  subprocess.check_call(["sscg"], cwd=certdir)
 
  httpd = HTTPServer(('', port), handler)
- ctx = ssl.create_default_context()
+ ctx = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
  ctx.check_hostname = False
  ctx.verify_mode = ssl.CERT_OPTIONAL
  # with newer Pythons this is ctx.minimum_version = ssl.TLSVersion.TLSv1_2