Fix unit tests with OS_DEBUG_MODE enabled

[sjanc]

Enabling OS_DEBUG_MODE results in some failures in unit tests (newt test all)

Error: Test failure(s):
Passed tests: [crypto/mbedtls/selftest encoding/base62/selftest encoding/base64/selftest encoding/json/selftest fs/fcb/selftest fs/fcb2/selftest hw/drivers/flash/enc_flash/selftest hw/drivers/trng/trng_sw/selftest hw/sensor/selftest kernel/os/selftest libc/baselibc/selftest net/ip/mn_socket/selftest net/oic/selftest sys/config/selftest-fcb sys/config/selftest-fcb2 sys/config/selftest-nffs sys/flash_map/selftest sys/log/full/selftest/align1_imghash sys/log/full/selftest/align2 sys/log/full/selftest/align4 sys/log/full/selftest/align8 sys/log/full/selftest/fcb2_bookmarks sys/log/full/selftest/fcb2_watermarks sys/log/full/selftest/fcb_bookmarks sys/log/full/selftest/fcb_bookmarks_mixed sys/log/modlog/selftest util/cbmem/selftest util/debounce/selftest util/rwlock/selftest cborattr/test nimble/controller/test nimble/host/audio/test nimble/host/test boot/boot_serial/test]
Failed tests: [fs/nffs/selftest sys/log/full/selftest/align1 sys/log/full/selftest/fcb2_align1 sys/log/full/selftest/perlogidx]