Only avocado.Test is of general interest to test writers. For that reason
the entire test module has been moved to the avocado.core namespace.

The recommended way to go about writing instrumented tests is something like:

   from avocado import Test
   class MyTest(Test):
      def test_foo(self):
         do_stuff()

Changes from v2:
* Fixed references in docstrings
* Fixed refrences of avocado.test in Writing Test Guide docs

Changes from v1:
* Fixed comment typo on commit message
* Fixed old reference of avocado.test (then test.Test) instead of
  avocado then Test in GDB docs.
Signed-off-by: NCleber Rosa <crosa@redhat.com>