Test reported file line / character position in unit tests

[Gama11]

Currently only the actual message is tested, which means we have no way to verify that the positions are correct. Maybe we could take some inspiration from clang-tidy here, they place comments directly in the code specifying expected message, line and character position, for example:

a = 123 * sizeof(s2);
// CHECK-MESSAGES: :[[@LINE-1]]:13: warning: sizeof() doesn't return the size

https://github.com/llvm-mirror/clang-tools-extra/blob/master/test/clang-tidy/misc-sizeof-container.cpp#L68-L69