Allow test seeding data to be intercepted/tweaked by providers

[roji]

As an example, PostgreSQL does not support storing a DateTimeOffset (with non-zero offset). Since test suites such as GearsOfWar and ConferencePlanner contain such data, Npgsql needs to override and modify the data; but seed data currently isn't factored so that this can be done easily.

• GearsOfWar, JsonQuery, ConferencePlanner
• ValueConvertersEndToEnd: TestValues
• BuiltInDataTypes
  • Tests like Can_insert_and_read_back_non_nullable_backed_data_types, Can_query_using_any_data_type_nullable_shadow which only work if all data types are supported exactly as-is.
  • AssertEqualIfMapped is private
  • Review the test suite in general

[lauxjpn]

We hit this one for Pomelo for the EF Core 6.0.1 update as well.

[roji]

/cc @maumar this is complicating my life also for JsonQueryNpgsqlTest. Nothing critical, just something to keep in mind.