[nrf fromtree] tests: boards: nrf: qdec: Add nRF54L20pdk to platform_allow

Enable test execution on nRF54L20pdk.
Overlay was already added.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>
(cherry picked from commit 0d7f6d1)

Author: nordic-segl

tests/boards/nrf/qdec/testcase.yaml

@@ -4,6 +4,7 @@ common:
     - nrf5340dk/nrf5340/cpuapp
     - nrf54l15dk/nrf54l15/cpuapp
     - nrf54l15dk/nrf54l15/cpuflpr
+    - nrf54l20pdk/nrf54l20/cpuapp
     - nrf54h20dk/nrf54h20/cpuapp
   integration_platforms:
     - nrf52840dk/nrf52840