fix: tests: Prevent hanging tests waiting for nonexistent device

Test hangs waiting for device with harness forever.
Such test should be filtered out and not executed,
as there is no such harness connected (not indicated in HW map).
Due to twister issue, harness filtering will work only when
'harness:' is provided explicit.

Signed-off-by: Piotr Kosycarz <[email protected]>

Author: nordic-piks

tests/subsys/zigbee/osif/serial/serial_async_api/testcase.yaml

@@ -2,5 +2,6 @@ tests:
   zigbee.osif.serial.async:
     platform_allow: nrf52840dk_nrf52840 nrf52833dk_nrf52833
     tags: osif_serial
+    harness: ztest
     harness_config:
       fixture: shorted_uart_1

tests/subsys/zigbee/osif/serial/serial_basic_api/testcase.yaml

@@ -2,5 +2,6 @@ tests:
   zigbee.osif.serial.basic:
     platform_allow: nrf52840dk_nrf52840 nrf52833dk_nrf52833
     tags: osif_serial
+    harness: ztest
     harness_config:
       fixture: shorted_uart_1