services/horizon: Improve sponsorship integration tests' running time…

… & flexibility. (#3090)

Refactors the sponsorship (CAP-33) integration tests:
 - Each test is independently runs as a sub-test of the  "TestSponsorships" test.
 - It introduces helper functions commonly used in each tests such as
   sandwiching sponsored operations, retrieving operations and effects, and
   confirming the existence of operations via `tt.Condition`.
 - `itest.MustGetAccount()` replaces instances of a manual force-retrieval of an
   account, and account/keypair management in general is simpler.
 - Running time is ~3x faster.

All of the actual _testing_ is still the same. Investigation on using 
`testing.T.Parallel` is in progress which should further improve running time.

Closes #3075.