Fail only printed a message to stderr and exited.
Instead, print it with g_error(), which also breaks
in the debugger and produces a core-dump.
Also, it constructed the message based on an unchecked
format string and constructed a format string dynamically.
Just don't do that.
Also add a comment that these macros are discouraged because
they are cumbersome to write (requiring a test-name and a failure
message).
Up to now, the "include" directory contained (only) header files that were
used project-wide by libs, core, clients, et al.
Since the directory now also contains a non-header file, the "include"
name is misleading. Instead of adding yet another directory that is
project-wide, with non-header-only content, rename the "include"
directory to "shared".
