selftests/net: Add trace events matching to tcp_ao

Setup trace points, add a new ftrace instance in order to not interfere
with the rest of the system, filtering by net namespace cookies.
Raise a new background thread that parses trace_pipe, matches them with
the list of expected events.

Wiring up trace events to selftests provides another insight if there is
anything unexpected happining in the tcp-ao code (i.e. key rotation when
it's not expected).

Note: in real programs libtraceevent should be used instead of this
manual labor of setting ftrace up and parsing. I'm not using it here
as I don't want to have an .so library dependency that one would have to
bring into VM or DUT (Device Under Test). Please, don't copy it over
into any real world programs, that aren't tests.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-8-05623636fe8c@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/tcp_ao/lib/utils.c

@@ -21,6 +21,32 @@ void randomize_buffer(void *buf, size_t buflen)
 	}
 }
 
+__printf(3, 4) int test_echo(const char *fname, bool append,
+			     const char *fmt, ...)
+{
+	size_t len, written;
+	va_list vargs;
+	char *msg;
+	FILE *f;
+
+	f = fopen(fname, append ? "a" : "w");
+	if (!f)
+		return -errno;
+
+	va_start(vargs, fmt);
+	msg = test_snprintf(fmt, vargs);
+	va_end(vargs);
+	if (!msg) {
+		fclose(f);
+		return -1;
+	}
+	len = strlen(msg);
+	written = fwrite(msg, 1, len, f);
+	fclose(f);
+	free(msg);
+	return written == len ? 0 : -1;
+}
+
 const struct sockaddr_in6 addr_any6 = {
 	.sin6_family	= AF_INET6,
 };