vboxsnapshotxmltest: Don't write to a file in abs_srcdir

vboxsnapshotxmltest: Don't write to a file in abs_srcdir

In the test, the snapshot XML is written into a file that's located
under:

  abs_srcdir/vboxsnapshotxmldata/testResult.vbox

However, the abs_srcdir doesn't have to be necessarily writable. It
should have been abs_builddir instead. Moreover, the label in the func
creating the file is called 'fail' while it fulfils the duty of
'cleanup' label.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>