config/test: add set_group_option to cbuilder

Before this patch, there was an inconsistency in config builder, one
could easily set options on global, replicaset and instance level, but
not on group level.

This patch adds a similar `set_group_option()` function that fixes the
inconsistency.

Part of #9895

NO_DOC=test helper update
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC