[Testing] Correct query plan for some operators (rollback) (#506)

To a good extent, this is a rollback of #395.

This investigation started with the query plan of `CacheProperties`
being wrong since... well, there was no `CacheProperties` operator in
the plan. The testing was happy that way, which was odd. It turned out
that it was because of the example query in the `Eager` operator, which
effectively _cleared_ the database, so that all examples afterwards ran
on an empty db. `CacheProperties` comes after `Eager` in the operators
page, so that the `CacheProperties` example ran on an empty db.

Summary of changes:
- Changed `Eager` example so that it deletes only _two_ nodes, rather
than _all_ nodes. The query plan is essentially unchanged, other than
indexes being used. Most notably, `Eager` is still triggered, presumably
as the db doesn't know upfront how many nodes have that property value
anyway.
- Fixing the above triggered a bunch of failures (for `CacheProperties`,
`EagerAggregation`, `Distinct`) , which are essentially the plans
changed in #395. I've restored them as they were before, but not altered
them otherwise.
- Given proper names to indexes/constraints, describing what they
_index_ rather than what they _test_.
- Moved the setup queries _inside_ the sections they belong to, rather
than _before_ the respective openings.
- Removed a setup block that dropped and recreated the exact same index.
- The `Delete` example was acting on `(me:Person {name: 'me'})`, which
has relationships attached to it, so an error is raised. Or rather, no
failure used to happen because, previously, the `Eager` example cleared
the whole db, so `DELETE me` was deleting a non-existent node, and now
the `Eager` example `DETACH DELETE`s the `me` node, so that it keeps not
existing. However, if for whatever reason we changed some of the
examples involved and it turned out that `me` existed with relationships
(as the page setup creates), then we'd get a failure. So I've changed
the `DELETE` example to act on `you`, rather than `me`, as `you` never
exists. There's also some perverse joy involved with pretending to
obliterate the reader, so I like it better.

Author: stefano-ottolenghi