selftests/net: move test out of Makefile into a shell script

Currently bpf test run from the Makefile. Move it out of the
Makefile to be run from a shell script to allow the test to
be run as stand-alone test, in addition to allowing the test
run from a make target.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Acked-by: David S. Miller <davem@davemloft.net>