Schema creation within `sqlserver__get_test_sql` macro is not concurrency-safe

[jay-taylor-ac]

This part of the sqlserver__get_test_sql macro that creates schemas is not concurrency-safe:

IF NOT EXISTS (SELECT * FROM sys.schemas WHERE name = '{{ target.schema }}')
BEGIN
  EXEC('CREATE SCHEMA [{{ target.schema }}]')
END

This results in a race condition when the number of threads are more than 1, which often results in the following error:

There is already an object named 'my_schema' in the database.

We have found this to be problematic during CI runs, where a new schema is created for each PR, often resulting in the first CI run failing due to the race condition and us needing to rerun the pipeline again.