Add `so project:test` custom command

[defunctl]

What does this do/fix?

Adds the custom command: so project:test, which runs all test suites so we don't have to run them all separately.

Tests

Does this have tests?

• [ ] Yes
• [x] No, this doesn't need tests because it's an so custom command.
• [ ] No, I need help figuring out how to write the tests.