Don't assert on error messages.

[tcard]

They may change (as this one has). That's what codes are for.

[tcard]

While the build still broken, the failures that this PR intended to fix does seem to have been fixed:

    ✓ JWT_and_realtime__client_initialized_with_a_JWT_token_in_ClientOptions__with_valid_credentials__pulls_stats_successfully (0.706 seconds)
    ✓ JWT_and_realtime__client_initialized_with_a_JWT_token_in_ClientOptions__with_invalid_credentials__fails_to_connect_with_reason__invalid_signature_ (0.115 seconds)
    ✓ JWT_and_realtime__when_using_authUrl__with_valid_credentials__fetches_a_channels_and_posts_a_message (0.776 seconds)
    ✓ JWT_and_realtime__when_using_authUrl__with_wrong_credentials__fails_to_connect_with_reason__invalid_signature_ (0.584 seconds)
    ✓ JWT_and_realtime__when_using_authUrl__when_token_expires__receives_a_40142_error_from_the_server (5.353 seconds)
    ✓ JWT_and_realtime__when_using_authUrl__when_the_server_sends_and_AUTH_protocol_message__client_reauths_correctly_without_going_through_a_disconnection (5.122 seconds)
    ✓ JWT_and_realtime__when_using_authCallback__with_valid_credentials__pulls_stats_successfully (1.130 seconds)
    ✓ JWT_and_realtime__when_using_authCallback__with_invalid_credentials__fails_to_connect (0.603 seconds)
    ✓ JWT_and_realtime__when_token_expires_and_has_a_means_to_renew__reconnects_using_authCallback_and_obtains_a_new_token (3.731 seconds)
    ✓ JWT_and_realtime__when_the_token_request_includes_a_clientId__the_clientId_is_the_same_specified_in_the_JWT_token_request (0.636 seconds)
    ✓ JWT_and_realtime__when_the_token_request_includes_subscribe_only_capabilities__fails_to_publish_to_a_channel_with_subscribe_only_capability (0.650 seconds)

The other failing tests seem to be inconsistently failing: they do pass in my machine, and they aren't even the same in the two build jobs that were run: #5326.1 and #5326.2. I think it's a safe bet to say they're unrelated to this PR.

So I think this should be merged.