Tests for Testament have to be in separated subdirectories. My guess, the documentation needs some updates, unless it is a bug. ;)
For example, if your main tests directory is `tests`, and you want to have test file `test0.nim` you have to put it in a path like `tests/subdir/test0.nim` Then run them with `testament run tests/subdir/test0.nim` or `testament pattern tests/**/*nim` to run all available tests.
