Acceptance tests: introduce BUILD_DIR and SOURCE_DIR

Acceptance tests: introduce BUILD_DIR and SOURCE_DIR

Some tests may benefit from using resources from a build directory.
This introduces three variables that can help tests find resources in
those directories.

First, a BUILD_DIR is assumed to exist, given that the primary form of
running the acceptance tests is from a build directory (which may or
may not be the same as the source tree, that is, the SOURCE_DIR).

If the directory containing the acceptance tests happens to be a link
to a directory, it's assumed to it points to the source tree
(SOURCE_DIR), which is the behavior defined on the QEMU Makefiles.  If
the directory containing the acceptance tests is not a link, then a
in-tree build is assumed, and the BUILD_DIR and SOURCE_DIR have the
same value.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Tested-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Tested-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20200317141654.29355-2-crosa@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>