Remove init file from dbt.tests directory

[gshank]

resolves #96

Problem

We install files from multiple packages in the dbt.tests hierarchy and that doesn't work if there's an __init__.py file.

Solution

Remove the file.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

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