[test][typetraits][warnings] Split the test for deprecated functionality in typetraits.hh into its own testcase.  That way I can simply disable deprecation warnings there.

There was a mechanism with apparently similar goal involving the define
DISABLE_DEPRECATED_METHOD_CHECKS.  However, I could not figure out how that
was intended to work, the only occurances of DISABLE_DEPRECATED_METHOD_CHECKS
were in typetraitstest.hh.  Probably you were supposed to set that define by
hand in your CPPFLAGS, which I consider unacceptable.