twister: reports: Long testcase.classname XML properties

The testcase.classname properties in `twister_report.xml` and
`twister_suite_report.xml` now contain the full testsuite name
(without its path prefix) instead of just two its dot-separated
leftmost components. This way `twister_report.xml` testsuite container
has no duplicate testcase elements with the same `classname` and `name`
properties executing with --no-detailed-test-id for a project with same
testcase names in its 'sibling' suites, for example`tests/kernel/sleep`.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>