Add testTargetDependencies to module reference

[marshall]

I stumbled upon this feature by accident searching high and low for a way to express a test dependency for a specific target, and I'd like to save others the time in the future by having it in the documentation :)

[marshall]

Not sure why the build failed, this is just a simple markdown modification. @autopulated who's a good person to ping about community contributions moving forward?

[autopulated]

@jaustin

[thegecko]

Thanks for the extra docs!