refactor(test/fixtures): use mergeFixtures

[OttoAllmendinger]

Instead of explicitly merging base fixtures and fork-coin fixtures, we want a general method that merges them. This reduces the risk of accidentally forgetting to merge fields.

The merge rules for merging a baseFixture and a forkFixture are as follows:

• For every property of forkFixture:
  • If forkFixture property is an object and baseFixture has a property with the same name that is also an object, merge both fixtures recursively.
  • If forkFixture property is an array and baseFixture has a property with the same name that is also an array, append elements to baseFixture.

Issue: BG-16466

[OttoAllmendinger]

superseded by https://github.com/BitGo/bitgo-utxo-lib/pull/71