Improve missing opportunity data errors to include specific numbers

[nickevansuk]

Referencing this discussion: #647 (comment)

❌ Missing Criteria: TestOpportunityBookableFree

• Opportunity Types: IndividualFacilityUseSlot, ScheduledSession
• Booking Flows: OpenBookingApprovalFlow, OpenBookingSimpleFlow
• Sellers: https://localhost:5001/api/identifiers/sellers/1
• Tests failing as a result: 72

Suggest this should be:

❌ Missing Criteria: TestOpportunityBookableFree

• Sellers: https://localhost:5001/api/identifiers/sellers/1
• Tests failing as a result:
  • IndividualFacilityUseSlot for OpenBookingApprovalFlow: 18
  • ScheduledSession for OpenBookingApprovalFlow: 18
  • IndividualFacilityUseSlot for OpenBookingSimpleFlow: 18
  • ScheduledSession for OpenBookingSimpleFlow: 18

As the Seller in use is configured in the test suite, there will generally be one of these. As the test data for sessions and facilities, approval and simple, are often provided separately, it likely helpful to have a quick breakdown of where the gaps are - so they can easily add more data if they need to without needing to inspect the results.

Additionally there will generally be a maximum of 4 permutations (though most implementations will likely only have 1 or 2), which makes the combinatorial explosion much less of a risk.