tests: Use '+=' to add additional tests, not '='

The recent commit that added the prom-env-test accidentially
overwrote the check-qtest-ppc-y, check-qtest-ppc64-y and
check-qtest-sparc-y variables instead of extending them.

Fixes: fcbf4a3c
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>