Acceptance tests: use relative location for tests

Acceptance tests: use relative location for tests

An Avocado Test ID[1] is composed by a number of components, but it
starts with the Test Name, usually a file system location that was
given to the loader.

Because the source directory is being given as a prefix to the
"tests/acceptance" directory containing the acceptance tests, the test
names will needlessly include the directory the user is using to host
the QEMU sources (and/or build tree).

Let's remove the source dir (or a build dir) from the path given to
the test loader.  This should give more constant names, and when using
result servers and databases, it should give the same test names
across executions from different people or from different directories.

[1] - https://avocado-framework.readthedocs.io/en/69.0/ReferenceGuide.html#test-id

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20191104151323.9883-5-crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>