Correct flaws in arc_summary[23] and their test.

The change correctly handles BrokenPipeError and improves the
associated tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12037
Closes #12036