Add QuicCryptoClientConfigTest.MultipleCanonicalEntries.

When working on cl/378863627 I originally wrote:

  auto it = canonical_server_map_.lower_bound(suffix_server_id);
  if (it == canonical_server_map_.end() || it->first < suffix_server_id) {
    canonical_server_map_.insert(...);
    return false;
  }

This is incorrect: lower_bound() never returns an iterator with key less than
its argument.  No tests caught this.  The statement should have been
  if (it == canonical_server_map_.end() || suffix_server_id < it->first) {
or simply
  if (it == canonical_server_map_.end() || it->first != suffix_server_id) {

This CL adds a test that would have failed with the incorrect statement.

PiperOrigin-RevId: 378922152