Check for NAN in CMake

[thetic]

This deprecates the HAS_NAN cache variable, that sets the CPPUTEST_NO_NAN variable that sets the CPPUTEST_NO_NAN macro that sets the CPPUTEST_HAS_NAN macro. Instead we use CMake to check for NaN support explicitly and use that result to set CPPUTEST_HAS_NAN scoped only to the tests.

• https://en.cppreference.com/w/cpp/types/numeric_limits/has_quiet_NaN

[coveralls]

Coverage remained the same at 98.515% when pulling 077848e on thetic:nan-check into a521a90 on cpputest:master.