[gpinitsystem2.0] Functional tests for expansion support

[srividyaky]

This commit adds functional tests for expansion support changes pushed in the PR https://github.com/greenplum-db/gpdb/pull/17335

Below changes are included:

init_cluster_config_validation_test.go - This contains tests related to the input expansion config validations, new functions GetDefaultExpansionConfig is added to create default config with expansion support along with config validaiton related tests

init_cluster_db_validation_test.go - This contains tests related to the database configuration checks after the cluster is successfully initialized with expansion support, it also included validation for spread and group mirroring

init_cluster_test.go: This includes expansion validation by passing config file with different file formats

init_cluster_suite_test.go: In this file check has been added to fetch the hostname