4.x: fix `getOrdinal` for system property and environment variable config sources

[barchetta]

Description

ConfigSource.getOrdinal() was returning 100 for the system property and environment variable config sources when the special property config_ordinal was not set in the config source (instead of the correct value of 400 and 300).

This PR corrects that and adds tests.

See MP Config Spec: https://download.eclipse.org/microprofile/microprofile-config-3.0/microprofile-config-spec-3.0.html#default_configsources

Fixes #8737

Documentation

No impact