twister: generate junit report with testsuites rather than platforms.

We have been generating reports with the platform being used as the
testsuite, ie. all testcases would land under one single big testsuite,
which is very difficult to use or read.

This change uses testsuites as intended and does that for each defined
testsuite (or scenario). Platforms are added as properties among other
things.

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