Add test builder

[worksofliam]

Adds a test builder to Source Orbit.

• Scans the project for .test files and marks them as 'test modules'
• Looks for all exports in the test modules and assumes they are test cases
• When -wt/--writeTests is provided:
  • the source will be written to disk
  • added the project dep tree
  • when combined with -bf make, will add a pseudo test target which calls the runner
• without -wt, then only validation of the tests are performed

To do

• [ ] Get a valid assert working for the test modules
• [ ] Provide better count in runner program
• [ ] Ensure exit code of not-zero when there is at least one failure