services/horizon: Update TestProtocol14StateVerifier to test LiquidityPoolEntry

[bartekn]

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.

Update TestProtocol14StateVerifier with Liquidity Pool entry to ensure state verifier works correctly. Rename TestProtocol14StateVerifier to TestStateVerifier and use the latest supported protocol version instead of 15.

Close #3839.

[tamirms]

this looks good. I think we can keep the state verifier integration test pretty basic (just create 1 of each ledger entry type) because we have https://github.com/stellar/go/blob/amm/services/horizon/internal/ingest/verify_test.go which which is a more extensive test of the state verifier

[bartekn]

Cool! I haven't noticed verify_test.go.