selftests/bpf: fix test_btf_dump with O=

test_btf_dump fails when run with O=, because it needs to access source
files and assumes they live in ./progs/, which is not the case in this
scenario.

Fix by instructing kselftest to copy btf_dump_test_case_*.c files to the
test directory. Since kselftest does not preserve directory structure,
adjust the test to look in ./progs/ and then in ./.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>