selftests/bpf: Do not use sign-file as testcase

The sign-file utility (from scripts/) is used in prog_tests/verify_pkcs7_sig.c,
but the utility should not be called as a test. Executing this utility produces
the following error:

  selftests: /linux/tools/testing/selftests/bpf: urandom_read
  ok 16 selftests: /linux/tools/testing/selftests/bpf: urandom_read

  selftests: /linux/tools/testing/selftests/bpf: sign-file
  not ok 17 selftests: /linux/tools/testing/selftests/bpf: sign-file # exit=2

Also, urandom_read is mistakenly used as a test. It does not lead to an error,
but should be moved over to TEST_GEN_FILES as well. The empty TEST_CUSTOM_PROGS
can then be removed.

Fixes: fc975906 ("selftests/bpf: Add test for bpf_verify_pkcs7_signature() kfunc")
Signed-off-by: Alexey Gladkov <legion@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/ZEuWFk3QyML9y5QQ@example.org
Link: https://lore.kernel.org/bpf/88e3ab23029d726a2703adcf6af8356f7a2d3483.1684316821.git.legion@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -88,8 +88,7 @@ TEST_GEN_PROGS_EXTENDED = test_sock_addr test_skb_cgroup_id_user \
 	xskxceiver xdp_redirect_multi xdp_synproxy veristat xdp_hw_metadata \
 	xdp_features
 
-TEST_CUSTOM_PROGS = $(OUTPUT)/urandom_read $(OUTPUT)/sign-file
-TEST_GEN_FILES += liburandom_read.so
+TEST_GEN_FILES += liburandom_read.so urandom_read sign-file
 
 # Emit succinct information message describing current building step
 # $1 - generic step name (e.g., CC, LINK, etc);