tests/bios-tables-test: Compiler warning fix

tests/bios-tables-test: Compiler warning fix

gcc 7.1.1 in fedora 26 moans about the:
   tables = g_new0(uint32_t, tables_nr)

because it can't convince itself that tables_nr is positive.
This is fallout from g_assert_cmpint no longer necessarily being
no-return;  replace it with a plain g_assert.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>