[nrf fromtree] tests: net: vlan: increase test stack for IPv6 send

On nrf52840dk/nrf52840 and nrf5340dk/nrf5340/cpuapp,
test_zz_vlan_embed_ll_hdr overflows its Ztest thread while the IPv6
UDP path builds a packet with an embedded link-layer header.

Use a 2048-byte stack for net.vlan.separate_net_buf.

Signed-off-by: Jakub Zymelka <jakub.zymelka@nordicsemi.no>

(cherry picked from commit 2cd64c5)

Author: jaz1-nordic

tests/net/vlan/tests.yaml

@@ -8,6 +8,7 @@ tests:
   net.vlan.separate_net_buf:
     extra_configs:
       - CONFIG_NET_L2_ETHERNET_RESERVE_HEADER=n
+      - CONFIG_ZTEST_STACK_SIZE=2048
   net.vlan.header_reserved:
     extra_configs:
       - CONFIG_NET_L2_ETHERNET_RESERVE_HEADER=y