Add test for warn, warnx, vwarn, and vwarnx with floating-point parameters

Similarly to what has been done for argp_error and argp_failure, this
patch patch adds new tests for the warn, warnx, vwarn, and vwarnx
functions.  The new tests use the format string to request the
conversion of long double parameters into string.  Currently, these
tests only check that the default format of the long double type works.
Future patches will extend the test for platforms that can have an
optional format for long double.

Tested for powerpc64le.

	* misc/Makefile (tests): Add tst-ldbl-warn.
	* misc/tst-ldbl-warn.c: New file.