*printf-posix tests: Fix for platforms where 'long double' == 'double'.

*printf-posix tests: Fix for platforms where 'long double' == 'double'.

* modules/snprintf-posix-tests (configure.ac): Require
gl_LONG_DOUBLE_VS_DOUBLE.
* modules/sprintf-posix-tests (configure.ac): Likewise.
* modules/vasnprintf-posix-tests (configure.ac): Likewise.
* modules/vasprintf-posix-tests (configure.ac): Likewise.
* modules/vsnprintf-posix-tests (configure.ac): Likewise.
* modules/vsprintf-posix-tests (configure.ac): Likewise.
* tests/test-snprintf-posix.h (test_function): Avoid 80-bit long double
tests on platforms where 'long double' is the same as 'double'.
* tests/test-sprintf-posix.h (test_function): Likewise.
* tests/test-vasnprintf-posix.c (test_function): Likewise.
* tests/test-vasprintf-posix.c (test_function): Likewise.