[test][usdt_probes][bionic] kill child manually

bionic has util-linux 2.31, `unshare` added support for `--kill-child` in 2.32.

This change remove the use of the argument `--kill-child` and instead kills the
child process manually before returning. While this is not going to take care
of all corner cases, it is probably more than enough for when the test is succeeding.

tests/cc/test_usdt_probes.cc

@@ -376,7 +376,7 @@ TEST_CASE("test probing running Ruby process in namespaces",
 
  SECTION("in separate mount namespace and separate PID namespace") {
  static char _unshare[] = "unshare";
- const char *const argv[8] = {_unshare, "--fork", "--kill-child",
+ const char *const argv[8] = {_unshare, "--fork",
  "--mount", "--pid", "--mount-proc",
  "ruby", NULL};
 
@@ -404,6 +404,7 @@ TEST_CASE("test probing running Ruby process in namespaces",
  std::string module = pid_root + "usr/local/bin/ruby";
  REQUIRE(bcc_resolve_symname(module.c_str(), "rb_gc_mark", 0x0, ruby_pid, nullptr, &sym) == 0);
  REQUIRE(std::string(sym.module).find(pid_root, 1) == std::string::npos);
+ kill(ruby_pid, SIGKILL);
  bcc_procutils_free(sym.module);
  }
 }