Add the ability to filter the tests according to their priority

In order to make it easier to perform testing campaigns at different level of details:
1/ For each test, it is now possible to define its priority (high/medium/low; default: low). The assignment of each test to a given priority level is subject to further evolution. 
2/ The launcher page (tests/index.html) now allows to filter the tests according to the chosen priority level. Also, sizing info (the number of tests) is shown.