Count backfill memory in the memory gate of the new runner

[sawenzel]

This fixes a problem in the memory admission of the new workflow runner and adds unit tests.

• The default tier ignored memory booked by backfill tasks, so --mem-limit could be exceeded.
• fits_default and mem_free_default now include the backfill memory.
• The backfill memory factor now defaults to 1.0 and can be set with --backfill-mem-factor.
• The simulator default is aligned with the runner.

[github-actions]

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