tests: refactor testSELinuxLoadDef

tests: refactor testSELinuxLoadDef

Since its introduction in
commit 907a39e735d256b8428ed4c77009d1f713aea19b
    Add a test suite for validating SELinux labelling

this function did not return NULL on OOM.

Since we abort on OOM now, switch testSELinuxMungePath to void,
return NULL explicitly on XML parsing failure and remove
the (now pointless) cleanup label.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>