selftest/bpf: Make crashes more debuggable in test_progs

selftest/bpf: Make crashes more debuggable in test_progs

Reset stdio before printing verbose log of the SIGSEGV'ed test.
Otherwise, it's hard to understand what's going on in the cases like [0].

With the following patch applied:

	--- a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
	+++ b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
	@@ -392,6 +392,11 @@ void test_xdp_metadata(void)
	 		       "generate freplace packet"))
	 		goto out;

	+
	+	ASSERT_EQ(1, 2, "oops");
	+	int *x = 0;
	+	*x = 1; /* die */
	+
	 	while (!retries--) {
	 		if (bpf_obj2->bss->called)
	 			break;

Before:

 #281     xdp_metadata:FAIL
Caught signal #11!
Stack trace:
./test_progs(crash_handler+0x1f)[0x55c919d98bcf]
/lib/x86_64-linux-gnu/libc.so.6(+0x3bf90)[0x7f36aea5df90]
./test_progs(test_xdp_metadata+0x1db0)[0x55c919d8c6d0]
./test_progs(+0x23b438)[0x55c919d9a438]
./test_progs(main+0x534)[0x55c919d99454]
/lib/x86_64-linux-gnu/libc.so.6(+0x2718a)[0x7f36aea4918a]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7f36aea49245]
./test_progs(_start+0x21)[0x55c919b82ef1]

After:

test_xdp_metadata:PASS:ip netns add xdp_metadata 0 nsec
open_netns:PASS:malloc token 0 nsec
open_netns:PASS:open /proc/self/ns/net 0 nsec
open_netns:PASS:open netns fd 0 nsec
open_netns:PASS:setns 0 nsec
..
test_xdp_metadata:FAIL:oops unexpected oops: actual 1 != expected 2
 #281     xdp_metadata:FAIL
Caught signal #11!
Stack trace:
./test_progs(crash_handler+0x1f)[0x562714a76bcf]
/lib/x86_64-linux-gnu/libc.so.6(+0x3bf90)[0x7fa663f9cf90]
./test_progs(test_xdp_metadata+0x1db0)[0x562714a6a6d0]
./test_progs(+0x23b438)[0x562714a78438]
./test_progs(main+0x534)[0x562714a77454]
/lib/x86_64-linux-gnu/libc.so.6(+0x2718a)[0x7fa663f8818a]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7fa663f88245]
./test_progs(_start+0x21)[0x562714860ef1]

0: https://github.com/kernel-patches/bpf/actions/runs/4019879316/jobs/6907358876

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20230127215705.1254316-1-sdf@google.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>