Add integration tests for etc-snapshot server flags and refactor /tests/integration/integration.go/K3sStartServer (#7300)

This adds integration tests for the following flags: "--etcd-snapshot-name","--etcd-snapshot-dir","--etcd-snapshot-retention","--etcd-snapshot-schedule-cron" and "--etcd-snapshot-compress". It also refactors K3sStartServer to stop applying strings.Fields() into inputArgs, so it can accept arguments that have space in their definition.
Signed-off-by: Ian Cardoso <osodracnai@gmail.com>