tests/acceptance: look for target architecture in test tags first

A test can, optionally, be tagged for one or many architectures.  If a
test has been tagged for a single architecture, there's a high chance
that the test won't run on other architectures.  This changes the
default order of choosing a default target architecture to use based
on the 'arch' tag value first.

The precedence order is for choosing a QEMU binary to use for a test
is now:

 * qemu_bin parameter
 * arch parameter
 * arch tag value (for example, x86_64 if "🥑 tags=arch:x86_64
   is used)

This means that if one runs:

 $ avocado run -p qemu_bin=/usr/bin/qemu-system-x86_64 test.py

No arch parameter or tag will influence the selection of the QEMU
target binary.  If one runs:

 $ avocado run -p arch=ppc64 test.py

The target binary selection mechanism will attempt to find a binary
such as "ppc64-softmmu/qemu-system-ppc64".  And finally, if one runs
a test that is tagged (in its docstring) with "arch:aarch64":

 $ avocado run aarch64.py

The target binary selection mechanism will attempt to find a binary
such as "aarch64-softmmu/qemu-system-aarch64".

At this time, no provision is made to cancel the execution of tests if
the arch parameter given (manually) does not match the test "arch"
tag, but it may be a useful default behavior to be added in the
future.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190312171824.5134-7-crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

docs/devel/testing.rst

@@ -740,7 +740,9 @@ A test may, for instance, use the same value when selecting the
 architecture of a kernel or disk image to boot a VM with.
 
 The ``arch`` attribute will be set to the test parameter of the same
-name, and if one is not given explicitly, it will be set to ``None``.
+name.  If one is not given explicitly, it will either be set to
+``None``, or, if the test is tagged with one (and only one)
+``:avocado: tags=arch:VALUE`` tag, it will be set to ``VALUE``.
 
 qemu_bin
 ~~~~~~~~

tests/acceptance/avocado_qemu/__init__.py

@@ -53,7 +53,12 @@ def pick_default_qemu_bin(arch=None):
 class Test(avocado.Test):
     def setUp(self):
         self._vms = {}
-        self.arch = self.params.get('arch')
+        arches = self.tags.get('arch', [])
+        if len(arches) == 1:
+            arch = arches.pop()
+        else:
+            arch = None
+        self.arch = self.params.get('arch', default=arch)
         default_qemu_bin = pick_default_qemu_bin(arch=self.arch)
         self.qemu_bin = self.params.get('qemu_bin',
                                         default=default_qemu_bin)