wic: debug mode to keep tmp directory

files in wic tmp directory can be usefull for debugging, so do not remove
tmp directory when wic create run with debugging mode (-D or --debug).

also update wic.Wic.test_debug_short and wic.Wic.test_debug_long to
check for tmp directory.

[YOCTO#14216]

Signed-off-by: Lee Chee Yang <Chee.Yang.Lee@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>