When building a test runner, don't remove the main function

The test runner works by generating its own main function and deleting from the AST any existing function tagged with `#[main]`. Now that we have a `#[main]` attribute, we don't actually need to remove the main function, just the attribute.

Note that this doesn't affect `main` functions that don't have the attribute, since that main is silently ignored when there's a `#[main]` somewhere. 
