tests: net: ieee802154: l2: Fix build warning with minimal libc

CONFIG_MBEDTLS_TEST enables mbed TLS self-test routines, which require
rand() function. As this function is not available by default with
minimal libc, it caused build warning on certain platforms.

As this feature is not really needed for 802.15.4 tests, simply remove
it from the project file to fix the issue.

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