tests: logging: fix build error

Currently we get the build error when building
'tests/subsys/logging/log_backend_uart':

error: static_assert expression is not an integral constant expression
BUILD_ASSERT(strlen(TEST_DATA) < SAMPLE_DATA_SIZE);
~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It can be easily workarounded by using sizeof instead of strlen
to calculate string size.

Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>