tests: bluetooth: tester: increase BT RX stack size for nrf54l

nrf54l15 needs more BT RX stack size for the following test cases:
MMDL/SR/GPL/BV-14-C
MMDL/SR/LHSL/BV-07-C
MMDL/SR/LHSL/BV-08-C
MMDL/SR/LXYL/BV-08-C
MMDL/CL/GPR/BV-08-C
MMDL/SR/LLN/BV-17-C

Signed-off-by: alperen sener <alperen.sener@nordicsemi.no>

Author: m-alperen-sener

tests/bluetooth/tester/boards/nrf54l15dk_nrf54l15_cpuapp.conf

@@ -1,3 +1,3 @@
 CONFIG_ZMS=y
-CONFIG_BT_RX_STACK_SIZE=4096
+CONFIG_BT_RX_STACK_SIZE=5120
 CONFIG_BT_MESH_ADV_STACK_SIZE=2048