tests: net: ipv6: Increase stack size for variable buf test

Increasing the stack size for variable_buf_size test. This avoids
stack overflow in various boards.

Fixes #84489

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

tests/net/ipv6/testcase.yaml

@@ -18,6 +18,7 @@ tests:
       - CONFIG_NET_PKT_BUF_RX_DATA_POOL_SIZE=4096
       - CONFIG_NET_PKT_BUF_TX_DATA_POOL_SIZE=4096
       - CONFIG_NET_IPV6_PE=n
+      - CONFIG_TEST_EXTRA_STACK_SIZE=1024
   net.ipv6.privacy_extension.prefer_public:
     extra_configs:
       - CONFIG_NET_IPV6_PE=y