tests: introduce preprocessed test cases

tests: introduce preprocessed test cases

The runtests script now always performs variable replacement on the
entire test source file before the test gets executed, and saves the
updated version in a temporary file (log/test[num]) so that all test
case readers/servers can use that version (if present) and thus enjoy
the powers of test case variable substitution.

This is necessary to allow complete port number freedom.

Test 309 is updated to work with a non-fixed port number thanks to this.