Rename _dbt_internal_test subquery alias

[jtcohen6]

Let's rename _dbt_internal_test here:

https://github.com/fishtown-analytics/dbt/blob/0f018ea5dd678d37668c819552328b3254c2f220/core/dbt/include/global_project/macros/materializations/test.sql#L4-L7

So that it doesn't start with an underscore. Believe it or not, this is a deal-breaker for some SQL syntaxes.

[vijaybalebail]

for Oracle , Nonquoted identifiers must begin with an alphabetic character from your database character set. Quoted identifiers can begin with any character.. Its recomended not to start with a _