securityselinuxlabeltest: Don't create dummy file in the srcdir

securityselinuxlabeltest: Don't create dummy file in the srcdir

At the very beginning of the test we check if the underlying
filesystem supports extended attributes as they are used to store fake
SELinux labels. In order to check that, a dummy file is created and
semi-random attribute is set. However, the file is created under:

  abs_srcdir "/securityselinuxlabeldata/testxattr"

which has two problems: abs_srcdir is not required to be writable, so
it should have been abs_builddir. The second one is - there's no
"securityselinuxlabeldata" folder under abs_builddir. The problem was
introduced in caf164f1.

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