Add tests with floating-point arguments for err* and verr* functions

Similarly to what has been done for argp_error, and argp_failure, as
well as for warn, warnx, vwarn, and vwarnx, this patch adds new tests
for the following functions: err, errx, verr, verrx, error, and
error_at_line.  The new tests check that the conversion of long double
variables into string works correctly on the default format of the type.
Future patches will reuse these tests for other formats that long double
can take.

Tested for powerpc64le.

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