Consolidate common test suite macros/functions to avoid code duplication

The MySQL Proxy test suite currently has a lot of code duplication, mostly around macros and common tasks, such as the macros used to declare testcases, certain convenience macros around asserts and setting up logging properly.
This task is about creating a common .h/.c file to be included in every testcases its use is applicable to, and cleaning up the existing testcases.
Optionally it would help to make the automake-based build system capable of only compiling the tests without requiring to run them all - a big win when working on a single test.