ZTS: Set $tmpdir before use in redacted_panic test

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

Set it as is done by the other redacted_panic tests.

Signed-off-by: Ryan Moeller <ryan@iXsystems.com>

tests/zfs-tests/tests/functional/redacted_send/redacted_panic.ksh

@@ -28,7 +28,7 @@ typeset ds_name="panic"
 typeset sendfs="$POOL/$ds_name"
 typeset recvfs="$POOL2/$ds_name"
 typeset clone="$POOL/${ds_name}_clone"
-typeset stream=$(mktemp $tmpdir/stream.XXXX)
+typeset stream=$(mktemp $TEST_BASE_DIR/stream.XXXX)
 
 log_onexit redacted_cleanup $sendfs $recvfs