There are a number of tests named explain<number>.js
> find . -name "explain[0-9]*.js"
./jstests/core/query/explain/explain5.js
./jstests/core/query/explain/explain6.js
./jstests/core/query/explain/explain4.js
./jstests/noPassthroughWithMongod/query/explain/explain2.js
./jstests/noPassthroughWithMongod/query/explain/explain3.js
./jstests/noPassthroughWithMongod/query/explain/explain1.js
We should rename and modernize these tests. explain1 and explain3 in noPassthroughWithMongod are more focused on behavior between yielding and DDL operations than explain, and may be convertible to FSM tests that QE could own.