[FTS] moss compaction unit test assumes one particular segment impl

These two tests:

=== RUN TestStoreCompaction
— FAIL: TestStoreCompaction (0.09s)
store_test.go:771: expected segment
=== RUN TestStoreCompactionDeferredSort
— FAIL: TestStoreCompactionDeferredSort (0.08s)
store_test.go:771: expected segment

Which are 2 variants of the same testStoreCompaction() function.

See: https://github.com/couchbase/moss/blob/master/store_test.go#L769

It would be nice if the test assertions were independent of the actual implementation. Or perhaps there could be another testableSegment interface that we hook into...