exp/services/ledgerexporter: create go integration tests for sub commands

[sreuland]

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

Added new go integration tests that verify ledger exporter 'scan-and-fill' and 'append' commands.

• uses quickstart for standalone core
• uses fsouza/fake-gcs-server
• uses support/datastore to verify exported files exist on gcs.

$ LEDGEREXPORTER_INTEGRATION_TESTS_ENABLED=true \
LEDGEREXPORTER_INTEGRATION_TESTS_CAPTIVE_CORE_BIN=stellar-core \
go test -v -race -run TestLedgerExporterTestSuite ./exp/services/ledgerexporter/...

Why

refactor from docker based testing infrastructure, follow standard convention in mono-repo of integration tests being next to unit tests, but enabled with env variable.

Closes https://stellarorg.atlassian.net/browse/HUBBLE-469

Known limitations

not fully parallelizable yet, some measure were taken for dynamic port usage of fake gcs server.