Unfortunately, "configuration" in your case includes not just Apache but any other application your using (PHP, MySQL, etc...), the OS and even the hardware and network configuration. Your test and production Apache configurations could be identical but if the directory permissions are wrong, or the network firewall isn't setup, or you forgot to update a hard coded IP address it either just won't work, or worse, will appear to work for a while and then break when you're not looking.
For testing purposes I wouldn't look at it from the standpoint of checking all the configuration besides perhaps a few critical parameters (installed applications, versions, any important hard-coded addresses, critical server parameters, etc...). After that I would just start hitting the web server with a variety of requests and making sure I get the expected response. I would test a variety of content types in a variety of locations. You don't want to just test loading
BigPageWithEverything.php as if that fails you don't know exactly what the problem is (though that kind of test can be useful in addition to other tests).
I haven't used any web server test framework beyond ab/siege but mainly for benchmarking purposes, which is another good area to get some numbers with while testing.