Fixes WOR-1059.

Introduces two new test handlers to workflow instances in vitest-pool-workers:

• getOuput(): retrieves the output of a successful instance
• getError(): retrieves the error info of a failed instance

I decided to introduce these as two separate functions rather than having one single function like getResult so tests end up more readable, and to not confuse between outputs that might have the same error structure ({name: string, message: string})

These are intended to be used after the workflow instance has reached either a complete or errored state, thus should be called after a waitForStatus({status: <status>}).

• Tests
  • Tests included/updated: fixtures updated to use it
  • Tests not necessary because:
• Public documentation
  • Cloudflare docs PR(s): [Workers/vitest-pool-workers] Add new test handlers to Workflows cloudflare-docs#27158
  • Documentation not necessary because:
• Wrangler V3 Backport
  • Wrangler PR:
  • Not necessary because: Workflows were not GA in v3

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