twister: rename TestSuite -> TestPlan

Change terminology and fix usage based on the current hierarchy. A
testsuite is each test application we have in the tree. The top class is
where we detect all the test suites and create the testplan to be run
later.
Content of testplan will have to split out, so it only deals with the
plan and not the overall execution of tests. This step will be done at a
later point.

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