selftests/bpf: convert send_signal.c to use subtests

Convert send_signal set of tests to be exposed as three sub-tests.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/send_signal.c

@@ -219,7 +219,10 @@ void test_send_signal(void)
 {
 	int ret = 0;
 
+	if (test__start_subtest("send_signal_tracepoint"))
 		ret |= test_send_signal_tracepoint();
+	if (test__start_subtest("send_signal_perf"))
 		ret |= test_send_signal_perf();
+	if (test__start_subtest("send_signal_nmi"))
 		ret |= test_send_signal_nmi();
 }