[sanitizer][test] Print result in getpwnam_r_invalid_user.cpp (#162217)

This test has been flaky failing on sanitizer-ppc64le-linux since at
least 9 days ago
(https://lab.llvm.org/buildbot/#/builders/72/builds/15257), but the
exact cause is unclear because the only output is that `assert(res == 0
|| res == ENOENT);` failed.

To aid debugging, this patch prints out the result of the `getpwnam_r`
call.

Author: thurstond

compiler-rt/test/sanitizer_common/TestCases/Linux/getpwnam_r_invalid_user.cpp

@@ -14,6 +14,8 @@ int main(void) {
   struct passwd *pwdres;
   char buf[10000];
   int res = getpwnam_r("no-such-user", &pwd, buf, sizeof(buf), &pwdres);
+  fprintf(stderr, "Result: %d\n", res);
+  fflush(stderr);
   assert(res == 0 || res == ENOENT);
   assert(pwdres == 0);
   return 0;