ZTS: Avoid unset $tmpdir in redacted_panic

The redacted_send tests make use of a $tmpdir variable, except in
redacted_send/redacted_panic the variable is never defined.

Use $TEST_BASE_DIR instead.

Clean up the stream file after the test.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #12455