Schema tests for sources are added to the manifest twice.

[iknox-fa]

Describe the bug

Tests that are applied to sources are added to the manifest twice. Once in the source patcher and once in the schema parser.

This wasn't caught before due to the early return in the exception method that should have been thrown when this occurs.

Steps To Reproduce

This behavior doesn't break functionality, however it's a performance hit and reduces our ability to throw helpful errors when duplicated tests are found.

To see what's actually happening start by commenting out lines noted in core/dbt/exceptions.py and run a dbt project that has tests applied to a source.

Expected behavior

dbt adds source tests to the manifest only once.

System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: ____________)

The output of dbt --version:

 0.20.0-b1

The operating system you're using:
macOS

The output of python --version:
Python 3.8.8