Use testcontainer for SQL Server tests by default

Currently [SQL Server must be installed before running our tests](https://github.com/dotnet/efcore/blob/main/docs/getting-and-building-the-code.md). Historically. people typically got this by installing Visual Studio with LocalDb. However, this is no longer a great option because:

- The version of LocalDb that ships with Visual Studio is currently, and is usually, an older version.
- This doesn't help people on non-Windows machines
- Manual installation and configuration of dependencies is error prone, with better solutions now available.

We should therefore start using a SQL Server testcontainer by default. I understand that this will not work on the C.I., so we should ensure that the C.I. continues to work either in the old way, or in some other way that is supported by the C.I. system.