Use continuous integration 
Requirements
All functionality in the source code MUST have automated tests.
Most code coverage tools have functionality to exclude specific lines if needed.
Contributions MUST pass all automated tests before they are admitted into the codebase.
The codebase MUST have guidelines explaining how to structure contributions.
Examples
The codebase MUST have active contributors who can review contributions.
Automated test results for contributions SHOULD be public.
The codebase guidelines SHOULD state that each contribution should focus on a single issue.
Examples
Source code test and documentation coverage SHOULD be monitored.
Testing policy and documentation for consistency with the source and vice versa is OPTIONAL.
Testing policy and documentation for style and broken links is OPTIONAL.
Also explain, and if needed provide links, how the testing is done so that the contributor can be efficient when making their contributions. I.e. list any linters that are used.