tests/sprintf: Remove tests with NULL FILE pointer

The C language says that use of a NULL FILE pointer with stdio functions is
undefined behavior. Let's just remove them instead of expecting the minimal
C library to exhibit a specific behavior in this case.

This also avoids problems when not using -ffreestanding as in that case,
the C compiler may generate warnings, or even cause undefined behavior on
its own.

Signed-off-by: Keith Packard <keithp@keithp.com>