This change splits the long-running change stream oplog rewrite test file `change_stream_null_existence_eq_rewrite_remove_test.js` into multiple smaller files:

• `change_stream_null_existence_eq_rewrite_remove_eq_ne_test.js`
• `change_stream_null_existence_eq_rewrite_remove_lte_gte_test.js`

The first file tests `$eq` and `$ne` rewrites, the second file tests `$lte` and `$gte` rewrites.

The split test files can be run in parallel in the CI, and as each file contains less tests, it does not risk running into test timeouts in the CI as easily as the combined file did.