services/horizon: Add txsub integration test

[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.

What

Adds sanity integration test for txsub. Add a test (submit_transaction_test.go) I forgot to include in #3653.

Why

While we have some unit tests for txsub we don't test the transaction submission endpoint during mirroring test. A simple integration test should at least catch the most obvious bugs.

Known limitations

[TODO or N/A]

[tamirms]

Don't all the existing integration tests exercise txsub because they all submit transactions to horizon?

[bartekn]

Correct but they all do it synchronously and submit only one tx per account at a time. I'm worried that changes like #3563 can introduce subtle bugs we won't catch using unit tests and simple testing.