Fix race within proxy integration test

src/envoy/http/jwt_auth/integration_test/http_filter_integration_test.cc:424: Failure
Value of: fake_upstream_connection_backend->close()
  Actual: false (The connection disconnected unexpectedly, and allow_unexpected_disconnects_ is false.
 See https://github.com/envoyproxy/envoy/blob/master/test/integration/README.md#unexpected-disconnects)
Expected: true
Stack trace:
  0x5555570aa6c1: testing::internal::HandleExceptionsInMethodIfSupported<>()
  0x5555570aa8f8: testing::Test::Run()
  0x5555570aabf1: testing::TestInfo::Run()
  0x5555570aae85: testing::TestSuite::Run()