selftests/bpf: Check for timeout in perf_link test

Recently perf_link test started unreliably failing on libbpf CI:
* https://github.com/libbpf/libbpf/actions/runs/11260672407/job/31312405473
* https://github.com/libbpf/libbpf/actions/runs/11260992334/job/31315514626
* https://github.com/libbpf/libbpf/actions/runs/11263162459/job/31320458251

Part of the test is running a dummy loop for a while and then checking
for a counter incremented by the test program.

Instead of waiting for an arbitrary number of loop iterations once,
check for the test counter in a loop and use get_time_ns() helper to
enforce a 100ms timeout.

v1: https://lore.kernel.org/bpf/zuRd072x9tumn2iN4wDNs5av0nu5nekMNV4PkR-YwCT10eFFTrUtZBRkLWFbrcCe7guvLStGQlhibo8qWojCO7i2-NGajes5GYIyynexD-w=@pm.me/

Signed-off-by: Ihor Solodrai <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 #include <pthread.h>
 #include <sched.h>
 #include <test_progs.h>
+#include "testing_helpers.h"
 #include "test_perf_link.skel.h"
+
+#define BURN_TIMEOUT_MS 100
+#define BURN_TIMEOUT_NS BURN_TIMEOUT_MS * 1000000
 
 static void burn_cpu(void)
 {
···
 	int run_cnt_before, run_cnt_after;
 	struct bpf_link_info info;
 	__u32 info_len = sizeof(info);
+	__u64 timeout_time_ns;
 
 	/* create perf event */
 	memset(&attr, 0, sizeof(attr));
···
 	ASSERT_GT(info.prog_id, 0, "link_prog_id");
 
 	/* ensure we get at least one perf_event prog execution */
-	burn_cpu();
-	ASSERT_GT(skel->bss->run_cnt, 0, "run_cnt");
+	timeout_time_ns = get_time_ns() + BURN_TIMEOUT_NS;
+	while (true) {
+		burn_cpu();
+		if (skel->bss->run_cnt > 0)
+			break;
+	        if (!ASSERT_LT(get_time_ns(), timeout_time_ns, "run_cnt_timeout"))
+			break;
+	}
 
 	/* perf_event is still active, but we close link and BPF program
 	 * shouldn't be executed anymore