tests: counter: kconfig: Fix debug log level enabling

COUNTER_LOG_LEVEL (defined in
subsys/logging/Kconfig.template.log_config) is derived automatically and
has no prompt. Assignments have no effect on promptless symbols.

Set COUNTER_LOG_LEVEL_DBG=y instead, which corresponds to
COUNTER_LOG_LEVEL=4.

Flagged by https://github.com/zephyrproject-rtos/zephyr/pull/20742

.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>