tests: net: Enable CONFIG_NET_BUF_VARIABLE_DATA_SIZE in some tests

CONFIG_NET_BUF_VARIABLE_DATA_SIZE option had not test coverage at all,
making in vulnerable for regressions. There is no dedicated test suite
to verify this option, but at least we can enable it in a few test
suites to make sure the configuration is functional.

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