tests: scheduling_group_test: verify that later() doesn't modify the current group
tests: scheduling_group_test: verify that later() doesn't modify the current group

It was suspected that later() modifies the current scheduling_group.
It does not, and this test ensures it will remain so.

Reviewed-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <adc2bcfad17c2d24e879764b4b9f798ea11186ee@scylladb.com>

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