vmtests: enable previously failing kprobe selftests

With fixes in selftests, these tests should now pass.
Also add ability to add comments to blacklist/whitelist to explain why certain
test is disabled.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>

travis-ci/vmtest/configs/blacklist/BLACKLIST-5.5.0

@@ -1,12 +1,16 @@
-attach_multi
-attach_probe
-bpf_tcp_ca
-fexit_bpf2bpf
-link_pinning
-perf_branches
-perf_buffer
+bpf_tcp_ca		# STRUCT_OPS is missing
+cgroup_attach_multi	# BPF_F_REPLACE_PROG missing
+fentry_fexit		# bpf_prog_test_tracing missing
+fentry_test		# bpf_prog_test_tracing missing
+fexit_bpf2bpf		# freplace is missing
+fexit_test		# bpf_prog_test_tracing missing
+link_pinning		# bpf_link is missing
+modify_return		# fmod_ret is missing
+ns_current_pid_tgid	# bpf_get_ns_current_pid_tgid() helper is missing
+perf_branches		# bpf_read_branch_records() helper is missing
 select_reuseport
-send_signal
+send_signal		# flaky
 skb_ctx
-sockmap_listen
-test_global_funcs
+sockmap_listen		# flaky
+test_global_funcs	# kernel doesn't support BTF linkage=global on FUNCs
+xdp_bpf2bpf		# freplace is missing

travis-ci/vmtest/configs/blacklist/BLACKLIST-latest

@@ -1,4 +1,2 @@
-attach_probe
-perf_buffer
-send_signal
-sockmap_listen
+send_signal		# flaky
+sockmap_listen		# flaky

travis-ci/vmtest/run_selftests.sh

@@ -21,12 +21,12 @@ test_verifier() {
 configs_path='libbpf/travis-ci/vmtest/configs'
 blacklist_path="$configs_path/blacklist/BLACKLIST-${KERNEL}"
 if [[ -s "${blacklist_path}" ]]; then
-	BLACKLIST=$(cat "${blacklist_path}" | tr '\n' ',')
+	BLACKLIST=$(cat "${blacklist_path}" | cut -d'#' -f1 | tr -s '[:space:]' ',')
 fi
 
 whitelist_path="$configs_path/whitelist/WHITELIST-${KERNEL}"
 if [[ -s "${whitelist_path}" ]]; then
-	WHITELIST=$(cat "${whitelist_path}" | tr '\n' ',')
+	WHITELIST=$(cat "${whitelist_path}" | cut -d'#' -f1 | tr -s '[:space:]' ',')
 fi
 
 cd libbpf/selftests/bpf