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.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>