Acceptance tests: drop usage of "🥑 enable"

The Avocado test runner attemps to find its INSTRUMENTED (that is,
Python based tests) in a manner that is as safe as possible to the
user.  Different from plain Python unittest, it won't load or
execute test code on an operation such as:

 $ avocado list tests/acceptance/

Before version 68.0, the logic implemented to identify INSTRUMENTED
tests would require either the "🥑 enable" or "🥑
recursive" statement as a flag for tests that would not inherit
directly from "avocado.Test".  This is not necessary anymore,
and because of that the boiler plate statements can now be removed.

Reference: https://avocado-framework.readthedocs.io/en/68.0/release_notes/68_0.html#users-test-writers
Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Caio Carrara <ccarrara@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20190218173723.26120-1-crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>

docs/devel/testing.rst

@@ -600,7 +600,6 @@ the ``avocado_qemu.Test`` class.  Here's a simple usage example:
 
   class Version(Test):
       """
-      :avocado: enable
       :avocado: tags=quick
       """
       def test_qmp_human_info_version(self):

tests/acceptance/boot_linux_console.py

@@ -18,7 +18,6 @@ class BootLinuxConsole(Test):
     Boots a x86_64 Linux kernel and checks that the console is operational
     and the kernel command line is properly passed from QEMU to the kernel
 
-    :avocado: enable
     :avocado: tags=x86_64
     """
 

tests/acceptance/linux_initrd.py

@@ -18,7 +18,6 @@ class LinuxInitrd(Test):
     """
     Checks QEMU evaluates correctly the initrd file passed as -initrd option.
 
-    :avocado: enable
     :avocado: tags=x86_64
     """
 

tests/acceptance/version.py

@@ -14,7 +14,6 @@ from avocado_qemu import Test
 
 class Version(Test):
     """
-    :avocado: enable
     :avocado: tags=quick
     """
     def test_qmp_human_info_version(self):

tests/acceptance/virtio_version.py

@@ -61,7 +61,6 @@ class VirtioVersionCheck(Test):
     same device tree created by `disable-modern` and
     `disable-legacy`.
 
-    :avocado: enable
     :avocado: tags=x86_64
     """
 

tests/acceptance/vnc.py

@@ -13,7 +13,6 @@ from avocado_qemu import Test
 
 class Vnc(Test):
     """
-    :avocado: enable
     :avocado: tags=vnc,quick
     """
     def test_no_vnc(self):

tests/requirements.txt

@@ -1,4 +1,4 @@
 # Add Python module requirements, one per line, to be installed
 # in the tests/venv Python virtual environment. For more info,
 # refer to: https://pip.pypa.io/en/stable/user_guide/#id1
-avocado-framework==65.0
+avocado-framework==68.0