bpf/selftests: Fix send_signal tracepoint tests

The send_signal tracepoint tests are non-deterministically failing in
CI. The test works as follows:

1. Two pairs of file descriptors are created using the pipe() function.
   One pair is used to communicate between a parent process -> child
   process, and the other for the reverse direction.

2. A child is fork()'ed. The child process registers a signal handler,
   notifies its parent that the signal handler is registered, and then
   and waits for its parent to have enabled a BPF program that sends a
   signal.

3. The parent opens and loads a BPF skeleton with programs that send
   signals to the child process. The different programs are triggered by
   different perf events (either NMI or normal perf), or by regular
   tracepoints. The signal is delivered to the child whenever the child
   triggers the program.

4. The child's signal handler is invoked, which sets a flag saying that
   the signal handler was reached. The child then signals to the parent
   that it received the signal, and the test ends.

The perf testcases (send_signal_perf{_thread} and
send_signal_nmi{_thread}) work 100% of the time, but the tracepoint
testcases fail non-deterministically because the tracepoint is not
always being fired for the child.

There are two tracepoint programs registered in the test:
'tracepoint/sched/sched_switch', and
'tracepoint/syscalls/sys_enter_nanosleep'. The child never intentionally
blocks, nor sleeps, so neither tracepoint is guaranteed to be triggered.
To fix this, we can have the child trigger the nanosleep program with a
usleep().

Before this patch, the test would fail locally every 2-3 runs. Now, it
doesn't fail after more than 1000 runs.
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230310061909.1420887-1-void@manifault.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -64,8 +64,12 @@ static void test_send_signal_common(struct perf_event_attr *attr,
 		ASSERT_EQ(read(pipe_p2c[0], buf, 1), 1, "pipe_read");
 
 		/* wait a little for signal handler */
-		for (int i = 0; i < 1000000000 && !sigusr1_received; i++)
+		for (int i = 0; i < 1000000000 && !sigusr1_received; i++) {
 			j /= i + j + 1;
+			if (!attr)
+				/* trigger the nanosleep tracepoint program. */
+				usleep(1);
+		}
 
 		buf[0] = sigusr1_received ? '2' : '0';
 		ASSERT_EQ(sigusr1_received, 1, "sigusr1_received");