selftests: bpf: test_kmod.sh: Fix running out of srctree

When running out of srctree, relative path to lib/test_bpf.ko is
different than when running in srctree. Check $building_out_of_srctree
environment variable and use a different relative path if needed.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717165326.6786-2-iii@linux.ibm.com