[Feature] Tests tags

This feature adds tags to test units.
Thanks to tags, the addition or deletion of a list of test unit is easier.

Current tests had been tagged with:
 - example: Miasm examples;
 - regression: Regression tests;
 - long: Tests known to be time consuming
 - llvm: Tests requiring LLVM

*test_all.py* has now an option to ommit some tags.
For instance :

    $> python test_all.py -t example           # Run only regression tests
    $> python test_all.py -t long              # Run only quick tests
    $> python test_all.py -t example,long,llvm # Run only quick regression tests, not LLVM dependent