Fix nvexec scheduler equality across stream priorities

[fallintoplace]

Summary

• include stream priority when comparing stream schedulers
• apply the same check to multi-GPU stream schedulers
• add regression coverage for equal resources, different priorities, and different contexts
• avoid constructing a second multi-GPU context in the regression test

Schedulers with the same hub but different priorities use different stream pools. They should not compare equal.

Tests:

• cmake --build build-cuda --target test.nvexec -j 8
• ./build-cuda/test/nvexec/test.nvexec --reporter compact

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[ericniebler]

/ok to test 6cf193a