sstable: Add tests for untested Writer code paths

Currently, there are unit tests for `Writer.Add` but not any of the
specialized exported methods on the Writer (eg. `Set`, `Merge`).
There are TODOs in the code for adding tests for these methods:

https://github.com/cockroachdb/pebble/blob/a4f88b796aeb07156512564261dd32ecadc80de6/sstable/writer.go#L617

We should add a unit test that calls all those untested methods for
completeness' sake. We could either modify existing unit tests to
sometimes call Set instead of Add, or we could add new unit tests
that check for equivalence between Set/Merge/Delete/RangeDelete and Add.
This could be a good starter / first issue.

Jira issue: PEBBLE-164