Update readme to display how to *run* tests

Modify configure.hpp.in to actually be useful for compile time
definitions and macros.
Add configure.hpp as header dependency for identity header
Add assert_is{_not_}_null exceptions for errors
Mark identity.fail as NORETURN
Remove skip.hpp's header dependency on configure.hpp

I should briefly note that while I am currently marking a few functions
as NORETURN, there really is no technical need to. It would most likely
be easier to remove the configure header altogether, and simply document
which functions will never return (but will throw or call std::exit)
Remove configure.hpp.in

This file is just unnecessary. If and when I need to use an attribute
such as [[noreturn]], it will be done when unittest v2 is released, and
it will be targeting a higher minimum compiler version, if at all.