changing 'configure' options for testing

[Daniel J. Luke]

The newest version of clamav uses cmake for builds. In the 'configure' stage, I have it disabling tests because otherwise it won't build without the test dependencies installed (check and pytest). 

Do we have a template or example of a canonical way to handle this? I don't see an obvious hook for when someone is running `port test` to change configure.args (I could, of course, add a post-extract/pre-configure and do some non-declaritive test to see if `port test` is being run and use that to branch - but that feels like a bad design choice).

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Daniel J. Luke