perf test: Mute test cases error messages if verbose == 0

Sometimes error messages in breaks the pretty output of 'perf test'.
For example:

  # mv /lib/modules/4.3.0-rc4+/build/vmlinux{,.bak}
  # perf test LLVM BPF
  35: Test LLVM searching and compiling                        :
  35.1: Basic BPF llvm compiling test                          : Ok
  35.2: Test kbuild searching                                  : Ok
  35.3: Compile source for BPF prologue generation test        : Ok
  37: Test BPF filter                                          :
  37.1: Test basic BPF filtering                               : Ok
  37.2: Test BPF prologue generation                           :Failed to find the path for kernel: No such file or directory FAILED!

This patch mute test cases thoroughly by redirect their stdout and
stderr to /dev/null when verbose == 0. After applying this patch:

  # ./perf test LLVM BPF
  35: Test LLVM searching and compiling                        :
  35.1: Basic BPF llvm compiling test                          : Ok
  35.2: Test kbuild searching                                  : Ok
  35.3: Compile source for BPF prologue generation test        : Ok
  37: Test BPF filter                                          :
  37.1: Test basic BPF filtering                               : Ok
  37.2: Test BPF prologue generation                           : FAILED!

  # ./perf test -v LLVM BPF
  35: Test LLVM searching and compiling                        :
  35.1: Basic BPF llvm compiling test                          :
  --- start ---
  test child forked, pid 13183
  Kernel build dir is set to /lib/modules/4.3.0-rc4+/build
  set env: KBUILD_DIR=/lib/modules/4.3.0-rc4+/build
  ...
  bpf: config 'func=null_lseek file->f_mode offset orig' is ok
  Looking at the vmlinux_path (7 entries long)
  Failed to find the path for kernel: No such file or directory
  bpf_probe: failed to convert perf probe eventsFailed to add events selected by BPF
  test child finished with -1
  ---- end ----
  Test BPF filter subtest 1: FAILED!
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1447749170-175898-6-git-send-email-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/builtin-test.c

@@ -226,6 +226,18 @@ static int run_test(struct test *test, int subtest)
 
 	if (!child) {
 		pr_debug("test child forked, pid %d\n", getpid());
+		if (!verbose) {
+			int nullfd = open("/dev/null", O_WRONLY);
+			if (nullfd >= 0) {
+				close(STDERR_FILENO);
+				close(STDOUT_FILENO);
+
+				dup2(nullfd, STDOUT_FILENO);
+				dup2(STDOUT_FILENO, STDERR_FILENO);
+				close(nullfd);
+			}
+		}
+
 		err = test->func(subtest);
 		exit(err);
 	}