[plt-scheme] getting coverage info for test-engine/scheme-tests

I'm trying the following (what I believe to be quite sane) development
methodology.

I'm writing some modules and in the directory where the modules live,
I've created a tests directory. Inside that directory, I have files
that require the modules in the containing directory, exercise them,
and then have test cases in the form of check-expects, etc.

How can I get coverage information from the files in the main
directory? I tried turning on the syntax highlighter, but I think that
only works in the current file, so it doesn't help if the tests live
in a separate file. I also tried following the instructions for the
errortrace module, but didn't get very far. In particular, I put

(require errortrace)
(coverage-counts-enabled #t)

at the top of a test file and

(test)
(annotate-covered-file "../filename.ss")

at the bottom, and just got an error. Unfortunately, I have no idea
what I should be putting to get the right information.

Any help?
Todd