scripts/sanitycheck: optimize disk usage for --save-tests

If we are doing a nightly build we can utilize a large (over 50G) of
disk space just to generate the list of tests to build.  We need to
optimize this so as we finish building the initial pass we clean up
as we go and only keep around the files we need (like .config,
generated_dts_board.conf, CMakeCache.txt, etc).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>