sanity: build testcase defconfigs for config_whitelist

Originally, the config_whitelist was done on the platform level
ignoring any config options in the testcase. This change generates
the defconfig for all relevant testcases that have config_whitelist
and applies filters based on the testcase defconfig.
If we have many testcases using config_whitelist it might be a bit
slower, but it is more accurate.

Change-Id: Ie1e871e85f05196d04419d6cf9a87f9927bfbfd1
Signed-off-by: Anas Nashif <anas.nashif@intel.com>