services/horizon: Revamps existing integration tests to run faster & be more modular.

[Shaptic]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

This PR revamps the claimable balance tests to run much faster, and also introduces some basic predicate tests.

The biggest change is that almost all tests are subtests, so the docker container is only created a single time for a large set of scenarios. This reduces risk of timeout errors, speeds up tests, and encourages better test practices.

Specifically,

• the "filter & query" test is now a series of subtests
• the "claim" tests are now shorter, more robust, and self-contained
• "helper" methods are more modular and flexible

New predicate tests will come in another PR. This also resolves #3000.

Why

Tests are good, fast tests are better ❤️

Known limitations

n/a