[nrf fromlist] tests: net: ipv6: Enable mbed TLS heap

mbed TLS MD operations, used by IPv6 Privacy Extensions, require access
to heap. This may vary depending on platform/mbed TLS variant used,
hence enable CONFIG_MBEDTLS_ENABLE_HEAP in tests to ensure mbed TLS has
always access to some heap memory.

Upstream PR: zephyrproject-rtos/zephyr#79303

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

tests/net/ipv6/prj.conf

@@ -37,3 +37,6 @@ CONFIG_MBEDTLS_LEGACY_CRYPTO_C=y
 
 # Increase the stack a bit for mps2/an385
 CONFIG_NET_RX_STACK_SIZE=1700
+
+# Make sure mbedTLS has access to heap
+CONFIG_MBEDTLS_ENABLE_HEAP=y