tests/sprintf: Skip UB-dependent tests on Picolibc
tests/sprintf: Skip UB-dependent tests on Picolibc

Passing NULL as the stream to stdio functions is "UB" (undefined behavior)
in the C standard. Picolibc crashes in this case, which is valid, but not
helpful. Skip these tests in this case.

Signed-off-by: Keith Packard <fd7f967895e9f35e58ec8a62a847a54d7fa7275f@keithp.com>

galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr