Fix commented-out database backend tests

Thanks a lot for your work on this!
Looking over this now, I see there are two separate issues that made the tests fail:

1. Though the filter tags added in #1390598: Add a way to easily identify facet filters inside the query are generally included in the D8 branch, it somehow seems to have slipped past us regarding the query's createFilter() method (in class and interface). That means, while everything else would (probably) be working fine, creating a filter with tags just fails silently, resulting in wrong OR facets (first batch of commented-out tests).
2. Then, the entity_test entity type has type now defined as a real bundle, not just any property. This, however, means that it can never be NULL – however, that's exactly what we need for the regression tests (second commented-out batch). It also influences the facets in the first batch, and would also be needed for a third commented-out batch (unrelated to facets) further down the page (around line 652).

So, the correct way to deal with this would be to fix the createFilter() oversight, add an additional property/field that can be NULL and then change the tests as little as possible.

Great job, thanks a lot! Looks very good and also passes both locally and here with the test bot.
Just one detail:

+++ b/tests/search_api_test_db/config/install/field.storage.entity_test.category.yml
@@ -0,0 +1,16 @@
+cardinality: -1

I'd say cardinality -1 would make more sense here, also to stay in line with the original.
For multi-valued fields, we already have keywords, after all.

The attached patch should fix this, just waiting for test bot confirmation.

OK, great.
Committed.
Thanks a lot again, really nice work!