Rails 6.1: Fix failing calculation tests

[aidanharan]

This PR fixes the following failing CI tests:

• CalculationsTest#test_select_avg_with_group_by_as_virtual_attribute_with_ar
• CalculationsTest#test_select_avg_with_joins_and_group_by_as_virtual_attribute_with_ar
• CalculationsTest#test_select_avg_with_group_by_as_virtual_attribute_with_sql
• CalculationsTest#test_select_avg_with_joins_and_group_by_as_virtual_attribute_with_sql

The tests needed to be coerced and re-implemented for the following reasons:

AVG() function
In SQL Server the AVG() function for a list of integers returns an integer. For other databases it would return a decimal. To get SQL Server to return a decimal result the values need to be casted before averaging. so need to cast values as decimals before averaging. So instead of AVG(credit_limit) AS avg_credit_limit we need to use AVG(CAST(credit_limit AS DECIMAL) AS avg_credit_limit.

SELECT columns must be in the GROUP clause too
SQL Servers queries cannot be:

SELECT companies.*
...
GROUP BY companies.id

They need to include all the company's columns in the GROUP clause like:

SELECT companies.*
...
GROUP BY companies.id, companies.type, companies.firm_id, companies.firm_name, companies.name, companies.client_of, companies.rating, companies.account_id, companies.description

LIMIT clause
SQL Server does not use the LIMIT clause but FETCH instead.