3.0 unit tests

I've updated the 3.0 unit tests so 'make check' should work again.

There was one thing specifically that I noticed while doing this - the
HttpMsg.cc file did a type-check in a method to see if it was an
instance of HttpReply. Its usually better to override the method in the
specific class that needs different behaviour - call the parent method
to get the basic behaviour. This reduces coupling between the classes
and makes it easier to write unit tests that link in with stub objects -
so we can test them without running up the full squid environment.

I'm going to spend some time this weekend working on reducing the
coupling in 3.0 a bit more.

Rob

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