`@testset`: with Xoshiro, restore Random.GLOBAL_SEED (#43188)
A `@testset` is supposed to restore the "global RNG state" as
it was before execution (so that they can be re-ordered easily, etc.)
Also, before a testset starts, the default RNG is re-seeded with the
"global seed" (to help reproduce test failures).
Before `Xoshiro` as the default RNG, the "global seed" was stored
within a `MersenneTwister` object. It was enough for a testset to
copy the default RNG at the start, and copy it back at the end.
But now the global seed is stored outside of the RNG, so it should
also be restored separately.
