This patch fixes and improves resolving of the test aliases (paths
relative to `data_dir.get_tests_dir`).

1. It removes the magic `.py` suffix, which prevented non-python files
   from being resolved (simple.sh was resolved to $tests/simple.sh.py)
2. It adds the feature to filter the class/method from the test
   reference by parsing $test_reference:$class.$method for implicit
   resolver. (previously this worked only for absolute path resolver)

Visible changes are:

* `avocado run passtest` => fails (used to work)
* `avocado run passtest.py` => works (did not work)
* `avocado run env_variables.sh` => works (did not work)
* `avocado run passtest.py:PassTest.test` => works (did not work)
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>