htools-excl.test: add test case for exclusion tags in hbal

In preparation for future modifications in the exclusion tags field, add a
test that verifies that exclusion tags are being honored: in a test cluster
with two instances of the same exclusion group in each node, hbal should
shuffle instances around to improve the score.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>