tests: Return EXIT_FAILURE/EXIT_SUCCESS instead of -1/0

tests: Return EXIT_FAILURE/EXIT_SUCCESS instead of -1/0

When using VIR_TEST_MAIN() or VIR_TEST_MAIN_PRELOAD() macros, the
retval of mymain() will become retval of main(). Hence, mymain()
should use EXIT_FAILURE and EXIT_SUCCESS return values for
greater portability. Another reason is that otherwise our summary
printing of failed tests doesn't work (see following commit for
more info).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>