sanitycheck: add subsets support

This new options makes it possible to only run a subset of the tests
which will allow us to run sanitycheck on multiple hosts and merge the
results into one report. This way we do not need to worry about
selecting specific architectures to be run on a certain host.

The option accepts a string value: x/y where x is the subset and y is
the total, so if we specify --subset 1/5, we will only run the first
fifth of the total tests, --subset 5/5 would only select the last fifth.

To get consistent results, the testcase instance list is now ordered,
to avoid duplications and have full coverage.

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