Change lookup_pushdown.js to allow disk use by default

In related ticket SERVER-63208, we are planning to specify allowDiskUseByDefault:false in this test. It was originally written to expect that disk use is prohibited by default, which means that when no index is available we fall back to a nested loop join rather than a hash join.

Given that we are making disk use allowed by default in this project, it probably makes sense for this test to use allowDiskUseByDefault:true and to expect to fall back to hash join for small collections rather than nested loop join.

CC eric.cox@mongodb.com