added optional msg parameter for assertion tests to provide additional

Hi,

I have added an optional msg parameter to the assertion functions to so developers can give addtional information about which test failed. 
e.g. if you have a test function A that calls a sub-routine B which does the actual asserting, PosInfo only points to the line in B that failed. But you won't know which line in A caused the error. If you pass PosInfo of B to your asserts you will know which line in A caused the failure, but not which specific test in B failed.
With my patch you can pass PosInfo from B and a message parameter to your assert and when it fails you get the line number from A and a prefix message from B telling you exactly where to look.