test_get_platforms_in_limit: refactor test for future extension
test_get_platforms_in_limit: refactor test for future extension

Refactor test to be more declarative and to allow more various input
configuration.

JIRA: OSBS-5923

Signed-off-by: Martin Bašti <388356a3851efe39ff0604de6d8fb2e16e10a0b7@redhat.com>

DBuildService/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor