Add testnet/pubnet sync, catchup (BoundedRange) and reingest integration tests

[bartekn]

Add integration tests that sync with the network: build state and ingest a few ledgers. This will help ensure that Horizon is able to sync and catch bugs like #3231 before release testing.

[bartekn]

To clarify, I think we should test 3 things:

1. starting from scratch to check if horizon can build state without a problems + a few ledgers,
2. starting from existing DB (start -> resume transition in FSM scenario). this would catch ingest/ledgerbackend: Fix off-by-one in CaptiveStellarCore.runFromParams #3231,
3. horizon db reingest so we are sure that reingesting works (I believe it doesn’t in the latest version because of ingest/ledgerbackend: Remove returning error on Stellar-Core process exit during catchup #3260).

[bartekn]

@2opremio @tamirms you asked why we should do testnet/pubnet tests and I answered that I was probably wrong and standalone should work too. That's true but what is the status of captive core tests in standalone? I'm looking at #3267 and realized that it wouldn't catch #3231 or #3260 because both issues were connected to captive core.

[2opremio]

Note that as soon as the integration tests can be run with captive core (which is almost there, I think) the tests at #3267 will be run with captive core too.

[bartekn]

OK! I just wanted to confirm we'll be able to have them before beta. If not, it would be great to have testnet/pubnet tests.