We currently don't enable lookup lowering in all-feature-flags BV because too many cases aren't implemented yet. Eventually, running the existing $lookup tests in classical BV and in all-feature-flags BV (and later, with the default flags) will provide the coverage but until then we should have a test that explicitly sets the flag and covers the basics of $lookup semantics in SBE.

Note: lookup_pushdown.js does set the flag explicitly and checks the explain output. It might be possible to test the semantics at the same time, but the test will likely need to be rewritten to be more data-driven.