tests/subsys/logging: Require a full C library

These tests all require a full C library to run. Replace the condition
checking for either newlib or picolibc with a check for
CONFIG_FULL_LIBC_SUPPORTED instead, then add CONFIG_REQUIRES_FULL_LIBC=y to
ensure a full libc is used.

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